A Zettelkasten is a way to organize and link together notes, where each note covers a single subject. It can be paper-based or digital, and together all notes form a hypertext system where notes can be linked to each other. This idea was first introduced by Niklas Luhmann in the 20th century as a paper-based hypertext system.
Since 2021, I have kept a Zettelkasten with Obsidian on my personal laptop. I have also synced these notes with Syncthing, Obsidian Sync and now iCloud. Keeping a Zettelkasten allows me to offload thoughts and have insights available as I work throughout the day. My Zettelkasten also forms part of my Mouse-less Development Environment on macOS, which allows me to open and query these notes as I need.
To try this idea, I first recommend you read this Guide by Zettelkasten.de, and then start with this minimal Obsidian Zettelkasten setup by Artem Kirsnov. Artem’s setup is ideal because it does not require extensive use of plugins and, in my opinion, has helped me a lot in focusing on writing rather than planning.