1 Minimal command of the skill. One knows the basics, but that's it. One can't train others at all.
2 Some more experience, but still novice level: a year or two in a craft or trade, or a skill one often uses but has never actively bothered practicing or received training in (in modern terms, "hobby" level). This is the very minimum at which one can start passing one's knowledge to others.
3 Apprentice level, or the level of skill for a secondary occupation (say, Leadership for a master craftsman, Cook for the grog who cooks the turb's meals while on a mission). This is the minimum skill at which one can train others somewhat effectively, and the very minimum at which one's tasks can be accomplished at a (barely) "professional" level (e.g. the minimum Magic Theory to set up an Hermetic laboratory).
4 Senior apprentice / junior journeyman level. This is the level of skill from a junior but competent professional (the top fighting skill of a green but well-trained young grog), or a mediocre one of greater age. It's difficult to earn one's livelyhood at this level of skill, unless one has a fairly strong natural talent in the area (characteristic +2 or higher).
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Solid journeyman level. A respected professional; people below 30 are unlikely to have skill levels above this unless they have a natural affinity for the skill. There's no shame in never raising above this level in your primary occupation, though many specialists eventually will. A modicum of talent in the area (characteristic +1 or higher) will allow one to earn one's livelyhood.
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Master level. If you are looking for someone who'll train you in a skill, this is the level you'd expect from him. Rules-wise, this is an important skill level because its a threshold for a number of crucial tasks. A scribe, bookbinder and illuminator at this level are required to produce books without Quality penalties (Covenants). A skill at this level will allow you to realize that a faerie's use of the ability is supernatural (RoP:F).
7-8. Senior master. This is the cap for most specialists in their profession, a level of skill rarely achieved before one's forties. If you get trained by someone with this level of skill, you are lucky. Most characters with this level of skill will feel there's very little that they can still learn in their occupation. Any character at this level of skill in his twenties is truly exceptional, and is likely considered a prodigy. The best professional a character personally knows (say, the Dean of one's guild, or the best swordsman among the grogs of an Autumn covenant) is likely to be at this level.
9-10. Paragon. Very few (mundane) characters reach this level of skill, and usually only after several decades of dedication to their craft. The best professional a character has ever heard of in a particular field (the best swordsman or blacksmith in the realm) is probably this level.
11+. Superhuman. A living legend. Most mundanes will simply think this level of skill is unattainable without supernatural means.
Note that there's a difference between what's typical and what's achievable. A character who obsessively spent since childhood all his free time (two seasons/year) practicing a favourite skill, to the exclusion of everything else, can reach a skill level of 11 by his fortieth year even without an Affinity or outside training in it. But most people lack that level of drive, and by the time one's skill is in the 6-7 range, the marginal returns of improving it further become very small indeed; and besides, to function effectively, even a specialist usually needs to round up his main skill with others if he wants to function effectively in life.