It's an Npub you can mention that provides some services for you. You tag it with a certain note in a certain format, and it replies with the output.

So an Npub might advertise itself as for example responding with a list of popular notes, and if you tag it then that's what it will respond with.

Opportunities to tag those npubs and receive those feeds, and also to offer your own feeds when tagged.

Scope for both for Johnny

(For more see NIP-90)