As a very, very new player to this, I can see that the Order forbids certain activities such as meddling with Infernals, and mundanes, and the church. So, what does that leave? A game where mages just act as recluse to make magic they can never use wouldn't be much of a game, so I wonder what are their motivations for leaning?
Also, what happens if the mundanes decide to kick out the mages from their land? Are the mages supposed to passively resist somehow?
Cheers!
The Oath is protection for other Magi. All of its restrictions on your actions are for the protection of other Magi from blow-back, with non-members having no actual rights or ability to bring charges.
Meddling is not the same as self-defense. If a group of mundanes decide to kick out the Magi then the Magi are perfectly within their right to "nuke them from orbit". Attacking a Magi is pretty much a 'get out of jail free' card in most cases (if it is a powerful mundane, you might have to explain things) and attacking a Covenant is even more of a free pass. Note that if the person attacking is really powerful like a King, Bishop, the Pope, etc then there are going to be questions after and you should always do everything possible not to kick-off a mundane/Magi war.
If some nasty group of mundanes goes after you, then generally you are ok with going after them. By this I mean groups that would attack other Magi no matter what you did and do not have support. The Barbary pirates are one such group. They are going to raid anyone and you are not going to piss off any rulers if you go after them. The varies church knightly orders are not, since if you actively go after them you will draw the ire of the Church down on Magi in general. Though if they are attacking you then you can of course defend yourself, just make sure to dispose of all witnesses and bodies.
In fact you can actually do some meddling as long as you do not draw anything bad down on other Magi. As long as a given mundane does not have any "power" then you can generally get away with doing whatever, as long as it does not upset a bunch of other mundanes and get them angry at the Order. Messing with powerful nobles, members of the church, or fae which could result in bad things happening to other Magi will get you in trouble. That is not to say don't do it, just make sure you cover your tracks if you do.
Also if you want to actively hunt Infernals then you are fine. Just don't make any deals with them (or don't get caught if you do).
So just ask yourself "does my action have a chance of causing blow-back on other Magi?". If the answer is yes, then make sure you don't get caught doing it. Because to get in trouble for an action, another Magi has to bring a charge against you. Once that happens it comes down to politics. If you are more liked then you can get away with pushing things. If you are disliked you can get in trouble even for self defense.
Remember it's all politics.
If you exterminate a whole castle but bribe/convince most of the Tribunal to absolve you, you're home free.
I meant you are a magus of good reputation who could never do such a horrible thing. You hold no responsability for that terrible accident and even collected a queen of vis to help your sodales who lost their mundane friends. Nobody important was hurt.
Some magi are recluses that just want to study their magic. They try anyway, because the outer world does not let them.
Other magi go outside and interact with the Church, nobles or mundanes, because they have to (to get something they need) or they want to (like the Jerbitons).
It is because the Oath exists that magi with powerful magic can still be interesting to play even when dealing mundanes with not so much defenses against them. How can one achieve what needs to be done, and not be caught/punished for it ?
Lovely answers. Thank you all so much for your time.
If possibly, you might want to have a look a the Guernicus part of HoH: TL, which includes a discussion of interpretations of the Code. 
What fuels drama in Ars Magica isn't whether a wizard can solve all their problems by magic, but what happens when the wizard does that. The Oath's restrictions about meddling with Infernals, mundanes, and the Church are indications of where you can expect to have interesting times if you solve those problems by blasting away with magic instead of taking a more thoughtful approach.
There's no interest in finding out whether a wizard can spend a few seasons in their lab to do whatever magic that they want. We all know that can happen, and that there's probably a half-dozen different ways for a Hermetic wizard to do any random thing besides. The point of the game is to find out what happens next.
If the mundanes kick your wizards from their lands, it'll be a different story if you all passively resist or just decide to pilum of fire anything that moves into your line of sight. Those sorts of decisions are where the game's narrative tension comes from, and that's what you're supposed to be doing in the game. Acting as a recluse to make magic isn't the game; it's the set-up for the game.
Ars Magica is essentially a game of super-powered humans in a medieval setting.
It's about why you do something, should you do something, how you do something, and what happens when you do something.
And, if you decide to break the rules, whether you can get away with it.
Also where it comes to the question of "do mundanes know about the Order?" keep in mind that if mundanes don't know that you are part of the Order, or better yet that you are a wizard, then there can be no blow back to the order for your actions, which pretty much gives you free reign so long as you maintain your secret.
A few points.
First, as the others on the thread have well-articulated, there are often exceptions to things the Code forbids, particularly for self-defense and defense of the Order writ large, and even outright Code violations still need to be discovered and brought to bear with sufficient political will to punish you. YSMV, but in my sagas the Code is often treated less as "rules to be followed" and more as "reasons to maintain secrecy from outsiders."
Second, there are many adventurous activities outside of what the Code forbids! You have total impunity to go mess with non-Hermetic magical groups. Within the Order, there are also lots of political groups, competitions, or mystery cults to join and/or advance your position within. There are magical locales both ancient and frontier-y to explore and plunder for secrets and cool creatures. For the arguably-standard "young magi found a Spring covenant" saga structure, there are all sorts of adventures to deal with establishing income and resource sources for the covenant, recruiting essential covenfolk and setting up structures to make them self-replacing, and dealing with whatever issues come pre-packaged into your covenant site (regiones, creatures from other Realms who live in or around the covenant, curses or other recurrent phenomena, difficult legal/political considerations with neighbors where dealing with mundanes without violating the Code can be genuinely difficult, etc).
But third, and I think the most important factor that hasn't really been delved into by other commenters and really should be stressed for new players: in Ars Magica, the reclusive parts of the game are fun. Ars Magica isn't like most RPGs, where you've got all the important stuff happening on adventures and the "downtime" mechanics are just to make you better at adventuring. Researching and developing magic in the lab is a fun part of the game, and the primary means by which many (though certainly not all) magi solve problems or pursue their goals. Doing long-term projects, both with and without magic, is (arguably) the core appeal of the game, and certainly the thing that separates Ars Magica from most RPGs.
Ars Magica puts you in the driver's seat of all the inventive capacities that are normally handwaved as the off-screen work of the greatest powers in a setting. It's a lot of fun to decide, say, that you want your covenant to be on a floating island and to actually work out step-by-step what the magical and logistical requirements are both to make that happen at all and then to make it sustainable for the people living there. And these fun-in-their-own-right processes are not mutually exclusive with adventuring, but often a source of adventures in their own right! A difficult magical task might require you to increase your Arts, which motivates you to go on an adventure to find and acquire a high-quality book on the relevant Art(s), and oftentimes these adventures will have costs or consequences that manifest in later adventures down the road, like from enemies you make or favors you end up owing.
So in short, don't write off the reclusive aspect of many magi as "not much of a game," and also don't worry too much because pursuing difficult magical projects will often motivate its own adventures.