To most people: early access to a game, a reason to ***** about it being buggy and generally an enormous ****-fest that they paid full price for so why the **** isn't it working? GAWD.
To sane people: early access to a Work In Progress version of a game. Basic functionality is there, but the game may be buggy or inefficient in some regards. The entire point of a beta is to iron out all as many of the kinks as possible before a game's official launch and to make sure that a good product goes out. It's akin to playing any Elder Scrolls game on launch week, except SI actually fix the issues that occur and you don't need to rely on extensive modding to have a playable game.