Also known as Zettelkasten, it is a system developed by Niklas Luhmamm of organising notes. It builds up into a Second Brain.
There are different types of notes in this system, enumerated here.
Fleeting Notes
Any idea that pops up in your head needs to be captured, while you’re in the market or reading or doing anything.
These are supposed to be worked on and new ideas inserted into The Slip Box in a day or so. Idea is to keep deleting these notes.
I use Obsidian on my phone for this. It syncs up to other places I take my notes in, using Syncthing
Literature Notes
The Slip Box
This is the place for notes. Here, the notes are linked to each other and new ideas are supposed to be emerge from the connections.
A note is supposed to have exactly one idea.