It’s a strange word, and like many fascinating words it’s German. Zettlekasten basically means “index card” and refers to a system of organising your thoughts onto cards.
The modern equivalent for us techie folk would be something like a wiki, which would allow for more efficient searching and linking between concepts. It’s a fascinating idea and something I’m going to play with.
The article has a lot more explanation than I can add, and it’s well worth a read.