A flat call is an unexpected call that is considered a way of slowing down the game, when a raise might have been the appropriate answer. The flat call can be a way of either playing cautiously when the other players, yet to play, might have better cards. It is also used as an effective way of bluffing others into believing that a player might not have bankable cards and is playing along only to see whether s/he can get something out of a decent, but not good, hand.
In the latter case, the player has to wait for others to fall for the bluff, and raise, so that all s/he needs to do is call. This way, the player does not project that s/he has a good set of cards and still get her/his way by increasing the amount in the pot.