Set
A set is three of a kind made from your pocket pair plus one matching board card, so both your hole cards are part of the hand.
A set is three of a kind where you start with a pocket pair and the board brings a third card of that rank. The key difference from trips is which cards are yours: a set uses both of your hole cards (your pocket pair) plus one board card, while trips means you hold just one matching card and the board pairs to supply the other two. Same hand on paper (three of a kind), but very different in practice.
It comes up when you've called or raised with a pocket pair and one of the community cards matches it. Sets are famously sneaky because nobody can see your pair, so the strength is hidden. Trips are easier for opponents to spot, since a paired board is visible to everyone and they can guess someone holds the matching rank.
For beginners, sets are some of the most profitable hands you can flop because they're disguised and tough to read. They make strong value bets and can let you trap, though you should still watch the board for straights and flushes that could beat you. Trying to flop a set with a small pocket pair is often called 'set mining.'
Example
You hold 7 of hearts and 7 of clubs, and the flop comes 7 of spades, K of diamonds, 2 of clubs, giving you a set of sevens. Compare that to holding A-7 when the board runs 7-7-K: that's trips (one hole card plus a paired board), not a set.