Timeboxes are used in agile approaches to project management. They represent a period of typically two to five weeks, sometimes referred to as sprints or iterations.
A number of objectives are agreed for each time box and these are prioritised using a nomenclature such as MoSCoW. Activities do not continue beyond the time box, even if the objectives have not been produced.
At the end of each timebox, anything that is incomplete (the backlog) is reviewed and prioritised along with the next set of objectives.
This approach places the emphasis on what scope can be achieved in the time available rather than how long it will take to achieve a certain set of objectives and encapsulates the agile approach to dealing with the triple constraint.