Check raising is the process, in poker, of checking on a betting round, waiting for other people to bet, then raising them on your next opportunity. You generally should do this when you are slowplaying, or representing a worse hand than what you are really holding -- this is the opposite of bluffing. The goal is to get the other players in for money (fattening the pot). The reason you'd do this is to force them to bet more strongly than they would if you had bet or raised. Of course, you lose the protection you might get if you had bet or raised initially, with your strong hand.
There are two schools of thought on check raising. Some people who play poker (preferably hold-em) for fun don't like the idea of check raising because it's an aggressive play and lures people into putting more money into the pot. Other people view it as a completely valid move and something that adds to the fun of poker.
It is also termed 'sandbagging'. Check-raising is sometimes banned from poker tables (particularly home games), because it erodes the balance of deceptive signals and legitimate signals in poker, which contributes to play for a longer period of time.