Heroic Inspiration

Spend Heroic Inspiration immediately after rolling any die to reroll it, and use the new roll. You can have only one Heroic Inspiration at a time. If you gain it when you already have it, you can give it to another player character in your group who lacks it. Your DM and some character features can grant it.

Source: supplied Player’s Handbook (2024), chapter 1 and Rules Glossary. This is a paraphrased quick reference; specific features can override the general rules.