what's so special about these spells? i mean it says they are not well understood by hermetic theory.
in game terms what does that mean?
In the case of Aegis of the Hearth, it means that you cannot invent a variant of the spell with a different Range, Duration or Target.
Pretty much the same thing with Whispering Winds.
In the case of Wizard's Communion, you can invent the spell with a different Duration (to use it with ritual spells), but that's about it. It just doesn't fit well with other guidelines.
What Arthur said.
With respect to the Aegis, I do allow size modifications to the Aegis, but it doesn't affect the final power level of the Aegis, so an Aegis 20, size +1 is still an Aegis 20 for purposes of determining penetration and its effects, but the final level of the spell is actually 25. The Boundary area of 100 pace diameter circle is fairly small. Although it might be part of the reason why covenants build towers and other tall buildings...
so i could not make a wizard's communion to cast spontaneous magic for instance. is that what it means?
to be honest this seems more like a balance reason than any complex metaphysics 
but i thought that learning foreign spells is impossible. let'say i find a hedge wizard's lab text. always assuming that the spell doesn't break any limits can i learn it?
even if it is sth that may normally require a minor or major breakthrough?
Yes, that's right. Take a look at the box of guidelines for MuVi. No MuVi spell can be used on spontaneous magic.
The other part was answered by callen, so I'm skipping to the next part. The only way that he could learn a hedge's spell is if he had the supernatural ability or abilities that are used in that spell. He might not even be able to piece together what the effect is, without those abilities...
so how was wizard's communion first learned? it's a spel lfrom the cult of mercury right? yet anyone meay learn it
I think a virtue was needed to learn it at all, probably Mecruian Magic as it still give the spell.
It isn't specifically described anywhere, I believe. My guess would be that it was an ability of the Cult of Mercury, which was partially adapted into Hermetic Magic as a spell.
Someone probably spent some time studying the Mercurian ritual "Wizard's Communion", and working out how to make it work with Hermetic Magic.
They managed to get it to work, as a Muto Vim spell, but they couldn't completely weave it into the Hermetic Structure, and therefore the spell exists but cannot be modified.