Single Opt-in: Single Opt-in means that when users submit their mobile number and email address to a signup form, they don’t have to confirm or verify their mobile number and email address to Subscribe or Opt-in. As soon as the user signs up, he or she is immediately added to the contacts module with Opt-In status TRUE.
Double Opt-in: In this case, users have to confirm their sign up. This means that when a user submits his mobile number and email address through the Sign Up form, they are sent a confirmation SMS/Email, where they must click a button or a link that confirms their desire to be added to your Contacts. Only after this confirmation, is the user added to Contacts with Opt-In status TRUE.