You can and should! Post it notes are good for stuff where you need to summarize symbol names that are sort of haphazardly distributed across a page. I'm a fan of a separate note section where I write plain English explanations of things, it's helpful in the moment for better appreciating and understanding something, and it's helpful six months in the future because it serves as bread crumbs on the path you first used to understand something, it feels very natural. If you wanted a spreadsheet you could have columns as name, equation, definitions, note, page reference. You might have to invent a name for some things but that's fine.