To say that an event has a timebox means that the event has a fixed duration that cannot be exceeded. The Scrum Guide states that "All events are time-boxed events, such that every event has a maximum duration.
Once a Sprint begins, its duration is fixed and cannot be shortened or lengthened. The remaining events may end whenever the purpose of the event is achieved, ensuring an appropriate amount of time is spent without allowing waste in the process."1 This means that the timebox sets an upper limit for the event, but the event can end sooner if the goal is met.
References:
* 1: The Scrum Guide2, page 10
* 2: The Scrum Guide