It's the act of lying down and throwing away your cards. You then withdraw from the current hand and lose the tokens you have committed.
Note that on tournaments lasting several days, players spend most of their time folding, because they expect good situations and good hands to take action. It is an action not to be overlooked. Beginner players tend not to fold enough and engage themself in many hands, they have a higher risk of losing the hands in which they’re engaged.
Hero fold is when you have a very good hand and you fold because you are convinced that your opponent has an even better hand.