This is a method of adding people to your mailing list. People subscribe to your mailing list and they double opt-in when they confirm their subscription with you. So, they will likely sign up on your site. Then they'll receive an email asking them to confirm their subscription. Their email is not added to your list until they've made that confirmation.
A method for verifying opt-in subscriptions which requires the subscriber to respond to an e-mail message after they have signed up to receive e-mail communications via an organizationâ€(tm)s website.
Process that requires no names on an e-mail list to take a specific action (such as clicking on to a confirmation page) to ensure that they have indeed opted in to receive messages as a part of the list; also called confirmed opt-in.