A bar, is to do with the timing. Say it's 4/4 timing, then four beats is one bar. So 12 bars is 12 sets of 4 beats. As for the 12 bar blues, it's just a standard blues progression that lasts 12 bars. I'm not sure why it's 12 exaclty, but all it is that the piece of music lasts for 12 bars before repeating or ending. Not the best of explanations, one o the other guys can probably do better.