Fold
To fold is to give up your hand and any claim on the pot, which costs you nothing further and returns nothing you have already put in.
Money already in the pot is not yours any more, and that is the hardest part of folding well. The only question is whether the next chip you put in earns more than it costs, and what you paid to get here has no bearing on that.
Folding is also the most common correct action in poker. A tight opening range folds most hands before the flop, and most hands that see a flop miss it, so a player who cannot fold is choosing to be wrong most of the time.
an example
your cards
You hold the worst starting hand in the game and face a raise. Against two aces it wins 11.61% of the time when all the money goes in before the flop, and against a normal raising range it does no better, so it goes in the muck.
This example is made up. The real one is in your own hand history: find fold in your own hands and see how often it came up and what it cost you.
related terms
Call
To call is to match the current bet exactly, which keeps you in the hand without raising the price for anyone else.
Muck
The muck is the pile of discarded cards, and to muck a hand is to throw it in without showing it.
Give up
To give up is to stop betting a hand you have been bluffing with and let the pot go, because another bet costs more than ...
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The full order of play, from the blinds to the showdown.