In a double opt-in process, customers need to additionally verify their mobile number/ Email address by clicking on a verification link sent to their mobile number/ Email address.
Email Double Opt-in Templates help you create the Email you would like to send for signup verification and save it as a Template. You can then set the Template to be auto-triggered from your signup form when you sign Contacts up via a double opt-in.
Suggested Reading: How to create Email Double Opt-In Templates.