Definition of slowplay
We talk about slowplay when you have a very strong hand that you play passively. There are several reasons for a slowplay. First, if your hand is too strong, it is better to play a slowplay to give your opponents a chance to hit something. If we have 77 in hand out of 7-7-2, it's clear that we won't get much action. Playing passively (slowplay) will be imperative.
The different reasons to make a slowplay
We can also do a slowplay because we are out of position against an aggressive player. If we have 77 out of 782 and we have the initiative against an aggressive player, it might be interesting to play passively to get him to bluff us. If we bet, we can convince him that we have a hand and he might fold, but if we bet slowplay, we might induce him to bluff us, in other words, to give us money that he wouldn't have given us otherwise. We can also make a slowplay to let our opponents understand that it's not necessarily because we play passively that we don't have a good hand. The slowplay has several virtues.