A bet meant to make better hands fold.
Whether it profits depends on how often the opponent folds. A ½-pot bet breaks even at 33% folds; a pot-size bet at 50%. Loose live players rarely fold, so bluff less.
At the table
A player who missed everything by the river bets half the pot to make a better hand fold. That is a bluff. If the opponent folds, the player takes the pot without showing their cards.