Most of the users on this site participate in course discussion forums. When you first access a forum Moodle may subscribe you to discussion threads you review. This means Moodle will keep you up to date with a discussion by sending you email copies of new forum posts. This is a feature many users find desirable, but some may find it overwhelming. Like me.
The user - you - can control how Moodle handles emails of posts by changing your preferences in the user profile page. Access it by clicking your name in the Participants block on the left of main course page or by clicking on your name in on of your own posts to a discussion thread.
The options for email copies include one email per new post, a full digest of all posts delivered once a day, or a digest of subject headings alone delivered once a day.
Of course, one can choose not to be subscribed to a forum at all; at which point no email copies of posts will be sent out. The user must visit the course to read any new posts.