NIP-51

Lists

NIP-51 defines portable lists, such as mute lists, bookmarks, pinned notes, and relay sets. Instead of keeping these preferences hidden inside one app, a user can publish them in a format other clients understand.

That makes your preferences more like luggage you can take with you. If you switch clients, the new app can read your lists and give you a familiar experience much faster.

Read more in the official 51.md NIP document.