Double opt-in is a system for adding new contacts to your email list that involves collecting an email address through a signup form and sending a confirmation email to that address in order to validate the contact information before adding it to the list.
This is also an opportunity to confirm subscriber consent which is an important aspect of GDPR-compliancy.
For double opt-in to work, a new subscriber needs to enter their email address into a signup form on your website or landing page. After, they'll receive a confirmation email. From there, they need to click on the confirmation link in the email.