Double Opt In is a process wherein a contact makes a request to opt in; however, by default they are opted out of marketing email communication until they receive an email and confirm their signup.
One advantage of the double opt-in process is that you'll know the email address is for a valid, monitored inbox. We recommend double opt-in for companies who aren't focused on audience growth, or who have had low open rates or abuse complaints in the past.
Companies may also decide to use a double opt-in process to satisfy regulations that may apply in the particular market or country in which they operate.
You can learn about setting up Double Opt In in our documentation here.