a bonus action is a swift, separate action a character can take during their turn, in addition to their standard action and movement, but only when a spell, ability, or other feature specifically allows it.
Examples include casting a bonus action spell like Healing Word, making an off-hand attack while wielding two weapons, or using a Rogue’s Cunning Action to Dash, Disengage, or Hide. A character only gets one bonus action per turn.