Folding means discarding the hand and leaving the pot to the other players. It costs nothing more but forfeits the chance of winning. Folding is a normal part of strategy, because many starting hands lose more over time than they win. If no bet has been made, a player can usually check instead of folding. Money already in the pot is gone once a player folds, which is why decisions should be based on the future cost and odds, not on what has already been paid.
Example
Facing a bet of 20 into a pot of 30 with a weak draw, a player folds and loses only the blinds or earlier bets already in.