All the half-baked ideas that pop into your mind. Have an easy way to capture them.
If I'm at my computer, org-capture. If I'm somewhere else, orgzly new note is where my fleeting notes go.
I think it's a thing from Ahrens and/or Zettelkasten.
These are all the ideas that pop into your mind. Always have a notebook or a note taking app with you to capture these. They can be very short—just one word for instance. The goal is just to remember them for a very short amount of time, until you can sit down and write a proper note