The best description I've found of it is this article: https://www.lesswrong.com/posts/NfdHG6oHBJ8Qxc26s/the-zettel.... Slightly long, but it completely describes how they work and how to implement them practically (on notecards). On the other hand, the articles on https://zettelkasten.de are pretty difficult to understand for the uninitiated.