When an email is selected as a "Marketing" Email Type, a line of text allowing the contact to unsubscribe from Marketing email messages will be added to the end of the email.
When they click this link, they will be taken to a web page confirming their action. You can check an individual contact's unsubscribe status on their contact profile, or show a full list by creating a contact segment.
If a contact wants to re-subscribe to your Marketing Emails, you can click the email icon next to "Unsubscribed" to send the contact an email to allow them to re-subscribe.
If you would like to customize any of the text for any of these messsages, you can do so by going to "Email Settings" in Admin.