You use boundary events to handle an event associated with an activity. A boundary event is always attached to an activity.
While the activity the boundary event is attached to is active (meaning the process instance execution is currently executing it right there), the boundary event is listening for a certain type of trigger. When the event is caught, the activity is either interrupted and the sequence flow going out of the event is followed (interrupting behavior) or a new execution is created from the boundary event (non-interrupting behavior).