Definition
Slow-playing (also called "trapping") is the strategy of under-representing a strong hand by checking or calling when you could raise or bet, with the goal of inducing opponents to bluff or build the pot on your behalf. The most common slow-play is "setting a trap" with a monster hand — checking the nuts on an early street hoping an opponent will bet on a later street.
Slow-playing is most appropriate when: your hand is so strong that you want opponents to build the pot (nuts on a dry board), betting would likely fold all worse hands but checking might induce bluffs, or you want opponents to make a hand strong enough to pay you off.
Slow-playing carries risk: opponents can catch up with draws, miss opportunities to build the pot if everyone checks, and can be "punished" in GTO play by opponents who recognize and exploit your tendency. Modern solvers show that slow-playing should be balanced with betting the same strong hands sometimes, to prevent exploitation.
Example
You flop the nut flush on a rainbow flop (A♠K♠2♠ — you hold Q♠J♠). Rather than betting into a pot where nothing connects, you check back. The turn brings a pair. Your opponent now bets into you. You raise. The slow-play worked.