You can enable double opt-in to add a layer of confirmation for subscribers to ensure that they'd like to hear from you. If you enable double opt-in and later decide to disable it, contacts who did not give confirmation while it was enabled will once again be eligible for email.
For example, a contact, John Smith, was not sent an email while double opt-in was enabled because he did not confirm opt-in. If you disable double opt-in, John will once again be eligible to receive emails going forward. He will not receive the email that was sent while double opt-in was enabled, but you can clone that email and send it again.