What is the "2Buss" ?

IT'S WHERE EVERYTHING GOES.

Basically on a console, the two signal paths that go to the speakers. It's what you're hearing, and the final two channels that end up being your mix. So when people talk about compressing the 2-buss (or stereo buss) they're talking about applying compression on the entire mix at once.
Also, just google stereo buss.