is bluffing, but in reverse. Since bluffing means betting strongly to insinuate a strong hand, slowplaying is betting weakly to suggest a weak hand. This tactic is used to get more money in the pot by allowing the opponents to thing your hand is weak, when in fact it cannot be beaten.
To check or call a bet with a big hand in order to win more money on future betting rounds.
Slowplaying is a great poker play, basically the opposite of bluffing. It means to check or bet weakly on a strong hand. The point of the slowplay is so your opponent can get a better hand but you know that your hand can not be beaten (the nuts). The purpose is to get more money into the pot by "bluffing" that you have nothing.