a mod is like a Leader, just with some added responsibilities.
Leaders basically help moderate the forum to keep it a safe and friendly place, and can take basic actions like closing/archiving/unlisting/merging topics and posts to help with this. these privileges aren’t available to other trust levels.
Mods, or Moderators, grant extra back-end abilities like approving flags, new users, and actual deletes. they also help out with things like tag group creation, username changes and badge granting – all jobs that need that backend moderator access
hope this helps! feel free to tag me with any further questions :))