Slow Playing is when you hold a strong hand but instead of betting you are checking because you don’t want to scare your opponents away. Perhaps they will catch something later or try to bluff you because they feel that you are weak since you are just checking (slow playing).
But i don’t really feel like it’s necessary for great card players to slow play their hands too often. It’s just not a needed part of the game. Sure, on occasion you’re going to flop a full house or four-of-a-kind and you’ll need to slow play the hand in order to let your opponents “catch up” to you and hit something on the turn or river, but even then, that’s only if you sense complete weakness at the table.
In general, a strong player should bet his monster hands and get paid off most of times.