To ensure CAN-SPAM compliance, Pardot requires that every email includes an unsubscribe link or email preference page link. When emails containing with the %%unsubscribe%% variable tag are sent using Pardot, Pardot replaces the tag with an unsubscribe link specific to the recipient. It will look something like this: http://www.site.com/unsubscribe/170bb3c3e580aeff2d05b2ee1dc53rab. The hash at the end of the link represents a specific prospect so Pardot can automatically unsubscribe the correct recipient. Similarly, if an email preference page link is included instead of the unsubscribe link, Pardot will rewrite the link in so it's specific to each recipient, and the email preference page will also include a link for the prospect to unsubscribe from all emails.
You can find your Unsubscribe Page by navigating to Marketing > Emails > Unsubscribe Page. By default, all unsubscribe pages contain the same message: "You have been successfully unsubscribed from email communications. If you did this in error, you may re-subscribe by clicking the button below." and thank you content: "You have successfully resubscribed to email communications."