Timeboxing means that you limit the amount of time that someone can work on a task to x days. Then, the project accept the output from that task even if it is not complete.
This is useful for example if a person’s task is to produce a document (say a draft user manual). If you give the author 5 days to prepare a draft (knowing that they could easily spend a month, if allowed to), then at least you get a draft document produced in 5 days. The cost of this approach is mostly low quality and possibly additional costs in the long-term.
(a definition provided by Cato Rocke-Meyer in the Critical Chain Yahoo Discussion Group)