Taken from PF2e Your heroic deeds earn you Hero Points, which grant you good fortune or let you recover from the brink of death. Unlike most aspects of your character, which persist over the long term, Hero Points last for only a single session.
The GM is in charge of awarding Hero Points. Usually, each character gets 1 Hero Point at the start of a session and can gain more later by performing heroic deeds—something selfless, daring, or beyond normal expectations. You can have a maximum of 3 Hero Points at a time, and you lose any remaining Hero Points at the end of a session.
You can spend your Hero Points to reroll a check. You must use the second result. This isn’t considered as an action, and you can spend Hero Points even if you aren’t able to act.