Fold
Folding means throwing away your hand and giving up any claim to the pot, ending your involvement in that round so you put in no more chips.
To fold is to surrender your cards and step out of the current hand. Once you do, you can't win the pot, but you also stop risking chips on a hand you don't like. Any chips you already put in stay in the pot. You'll often hear it called "laying it down" or "giving up" the hand.
Folding only comes up when there's a bet or raise in front of you and you'd rather not match it. Facing a bet, your choices are to call, raise, or fold. If there's no bet to you, you can't fold for free; you simply check (or bet) instead. This holds on every street: preflop, flop, turn, and river. If everyone folds to one player, that player wins the pot right away without showing their cards.
For beginners, folding is your most-used and most underrated move, not a sign of weakness. Good players fold a lot of starting hands and let go of weak ones rather than chasing. The flip side: when you do put chips in, you're giving opponents the chance to fold too, which is where ideas like fold equity and bluffing come from.
Example
You're on the button with 7-2 offsuit, the under-the-gun player raises to 3 big blinds, and two others call; rather than cold call with the worst starting hand in Hold'em, you fold and wait for a better spot. Later you hold A-K, miss the flop completely, and face a big bet on the river, so you lay it down instead of paying to see a likely-better hand.