A fold in poker is an action by which a player gives up on their hand and any chance of winning the pot.
When a player folds, their hand is dead, and they no longer participate in any future betting action. After folding, they can no longer win the pot, but they also can’t lose any additional money in that particular hand.
In a live game, you can verbalize your decision to fold, and once you say the word, that decision is binding. You can’t change your mind and decide to take a different action and suddenly snap call after you verbally announce a fold.
Another way to demonstrate your desire to fold is to toss your cards into the muck or simply push them face-down toward the dealer. The dealer will then take your cards and put them in the muck, finalizing the fold.