'Submission Limit' determines the number of times a user is allowed to submit a form which can be important for signups to events for entries to competitions.
Multiple: The default option is to allow for multiple forms to be submitted this is standard for forms like Contact Forms and for general enquiries.
1 per Email Address: As the name suggests this option only allows one submission per email address and is useful for Competitions, Giveaways, and Signups.