Here, you can define conditions to validate a user's submission. If these conditions are not fulfilled, the submission won't be saved, and the user will be asked to modify their submission.
The less good part : if you mess around and end up breaking your form, don't come whining ! (Don't worry, you'll 'only' have broken your own form, the worst that can happen is having to start over a new one :)
You must insert at least ONE form field before clicking « Submit ». If your form remains empty, it will be considered as a potential spam, and you might loose your work.