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.
SMS Double Opt-in Templates help you create the SMS 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 an SMS Double Opt-in Template?