I googled for ten minutes, still did not find answer. What does 2.1 mean? I assume THX is just a certification and doesn't really mean anything in and of itself?

What does 5.1 do that 2.1 doesn't? Five speakers instead of two?

2.1 is fake surround - two speakers and a sub woofer. There are no speakers for the Center or Surround channels. It uses some audio processing to make it not sound like a stereo - but it sure ain't surround.

This refers to the actual number of speakers plus the subwoofer: a 5.1 system has five individual speaker units plus a subwoofer (total of six speaker units). A 2.1 system has two speaker units plus a subwoofer (total of three speaker units). Just some shorthand to make audiophiles feel special.