Usernotes are little text notes that Admins/mods can add to user accounts. For example, if a user is constantly picking fights with other users, a mod could add a note to his account saying "Known trouble maker - look at thread history - 1 more warning then ban?" - only other mods/admins could read this note.
Another example - an admin might see the good work a user is doing in helping his fellow members, so will put a usernote on his account saying "Helpful user, possible new moderator".
It's just easier than writing things on post-it-notes, plus all other mods/admins can see the notes (if you give them permission to)
Thanks,
Alan.