No digest (single email per forum post) - you will be emailed a separate emailnotification for every single forum post to a forum you are subscribed to. This can spam your email if you are subscribed to a busy forum, or many forums.
Complete (daily email with full posts) - you will receive one email per day with all that day's forum posts compiled on it, in full.
Subjects (daily email with subjects only) - you will receive one email per day with all that day's forum posts compiled on it in list format, showing only the subject lines. Click the subject lines to go to that forum post to read it (you may be
required to log into Moodle to see the forum).