A timebox is a period of fixed maximum duration in which team activities may be carried out. In Scrum, there are five timeboxed events: Sprint Planning, the Daily Scrum, the Sprint Review, the Sprint Retrospective, and the Sprint itself which is a timebox containing all other events. In Agile practice, the delivery of value should not be put in unnecessary delay. Brisk timeboxing can help, since events are often subject to the law of diminishing returns.
For example, it’s unlikely that spending two working days on Sprint Planning will be twice as effective as one working day. It is believed that the purposes of Sprint Planning may be reasonably accomplished in 8 hours or less. Any longer is not likely to improve the quality of the activity, and it would be better to commence work and learn from actual experience, inspecting and adapting the Sprint Backlog accordingly.