Progressive summarization is a note-taking technique that involves repeatedly refining and summarizing information to create a hierarchy of important points.
The layers of progressive summarization.
This technique depends on the idea of Just-In-Time project management or learning.
The idea is that most information won't ever be relevant again. Therefore, don't spend a ton of time processing notes at the start because you don't really know yet, what will be useful. It's just extra work that will probably prevent you from getting the real work done.
Instead, notes are easily available via search. So when you're working on a project, you should be able to find what you're looking for. The goal with progressive summarization is to make things easier for future you on the notes that are most relevant to your work.
Every time you revisit a note, you put in a little effort to make it more valuable to future you. The way Tiago Forte does it is illustrated above. Not every note will get through all the levels. You only move a note forward a bit each time you refer to it in your work.
That way, the only notes that get through all the levels are the most important ones.