To test it: Submit the form successfully once with an e-mail address (could be a demo/fictional one), then try to submit the form again using the same e-mail. You will notice the error notice, letting you know that the e-mail already exists in the records.
This is an example of the “Unique Entry per Submit” Feature. For example, this can be used if you want to have only 1 email address submitted. Could be for a Contest or an E-Mail List for Newsletters etc.