What are the different types of opt-ins?

You can find the Email Opt-in field in a contact record.
Here are the different values it can take:

• None: Set automatically when a contact’s primary email is edited or changed
• Single opt-in (user): Set manually by a user when a contact agrees to receive marketing emails or set automatically when a contact is created
• Single opt-in (web form): Set automatically when contacts submit a web form with the email marketing checkbox selected
• Double opt-in: Set automatically when a contact clicks the double opt-in email link
• Opted out (user): Set manually by a user when a contact requests to opt out of email marketing
• Opted out (contact): Set automatically when a contact unsubscribes from their email preferences page

Note: According to GDPR, you must essentially have double opt-in from your contacts to send them marketing emails.