In poker the term 'slowplaying' refers to playing a particular hand much more passively than you usually would in an attempt to deceive your opponent into thinking your hand is much weaker than it is.
Used sparingly it can be a very powerful tool that can increase your profits when you play poker, but if overused or used in the wrong circumstances it can be a high-risk and costly move to make.
The best time to slowplay is when you have a strong hand on a board that contains no or few draws and your opponent is someone who is aggressive and bets frequently. For example, in a cash game you decide to limp with a pair of black nines and an aggressive player makes his standard raise, which you call.
The flop comes down 4d 9h Ks. Now is a perfect time to slowplay. If you check with your set of nines, your aggressive opponent is almost certainly going to bet to represent the king even if it has not helped him. When he bets you can flat-call and hope to repeat the same action on a safe turn. Alternatively, you could check-raise him and win at least the extra bet he has just put into the pot, something he might not have done if you had lead out. If he is aggressive or reckless enough, he may even come over the top of your check-raise with top pair and commit his entire stack as a huge underdog, which is obviously a great situation for you to be in.
Whenever there are multiple players in the hand or the flop contains a big draw it is not advisable to slowplay your hand. Imagine you have a pair of black nines again but this time you and three others see a flop reading 9h Th 5c. While there is a high chance you have the best hand right now, there are a number of possible holdings your opponents could have that have the potential to outdraw you by the river.
It's possible your opponents could have hands such as eight-seven, queen-jack or a flush draw, so you should bet out to protect your hand rather than try and trap other players as you could give away a free card and allow someone the chance to outdraw you.
If you are in any doubt it is probably best not to slowplay your hand as it is much better to win a small pot then lose a large one.