That means they will sit down for fifteen minutes and start typing/writing furiously until their timer goes off and they have the next scene or part of a chapter.
Some authors write in writing sprints. That means they will sit down for fifteen minutes and start typing/writing furiously until their timer goes off and they have the next scene or part of a chapter. These are people who work best with many small chunks of writing time over the course of their day instead of large blocks and can easily switch their focus from one task to the next.