Set how long you’ll work on a task. Work on it only that long. Time blocking with a deadline.
Timeboxing is essentially Time Blocking, but where you further stipulate that the chunk of scope you’re dedicating a block of time to WILL BE DONE by the end of that block. This ensures that you’re focused, distraction-free, on the task at hand, AND that you won’t let effort on any given task crowd into time you’d previously planned to use for other work.
A Timeboxed schedule is what Cal Newport (author of Deep Work) credits for his ability to get so much done in a week.
Why
The purpose of this is to combat perfectionism. It is a form of applying the Pareto Principle to achieve the vast majority of what needs to get done in the minimum amount of time.