What is this? It is about constantly decomposing bigger tasks into smaller ones in order to always have something easy to do. This method may not be useful to everyone, but for some of us, being in front of a big task makes us freeze, and even possibly pushes us toward doing something else because the task seems too intimidating.
So by decomposing the big task into really small ones, it becomes a lot easier to choose one and do it right away, and then do another one, and another one, and so on. And if some small task still seems like too much, decompose it into smaller tasks, small enough that they seem trivial. That way we can finally get some progress on complicated stuff.
It is a way to alleviate the fear of doing something that is too hard or too long. And it can also give a stronger feeling of accomplishment, because instead of having simply worked on some vague and small part of a big project, we see it as having completed many tasks about the project. The advancement is much clearer that way, and it feels motivating to know that actual parts of the project are done, rather than just knowing that we put some incalculable work into it.