The Suppressions list is a list of email addresses, to whom - for some reason or another (detailed below) - marketing emails will not be sent. (See this article about marketing status in general).
Here are the reasons an email address can end up suppressed:
If the contact unsubscribed from your list by clicking "Unsubscribe" in a Remarkety email, they will automatically be suppressed. You as an admin cannot remove this suppression... The only way is for the customer themselves to re-subscribe via a signup form, popup, etc.
The email was manually added by a Remarkety user to this list, either through adding an email in the Remarkety app or by uploading an "Unsubscribe" list. This suppression can be removed by a Remarkety user.
The email has bounced several times, or the ISP has informed us that the mailbox is invalid or does not exist.
The email bounced but this might be a temporary glitch. This suppression is usually safe to delete after a while.
The recipient has marked at least one of your emails as spam. As a precaution, we will suppress this user so that they do not continue to damage your email reputation.
This suppression is in place for recipients who've signed up to your list but haven't confirmed their email ("double opt-in"). Until they do so, this suppression is in place. Mind that "double opt-in", although sounding cumbersome, is a sound way to keep your list clean from bots and unengaged customers.
The email's domain should never receive any marketing emails. This can happen when you use your eCommerce to sell on other channels, such as Amazon or eBay. You usually get an email address that belongs to the marketplace and hides the real customer's email, and the platforms usually restrict you from sending anything but basic followup emails. Remarkety helps you manage this issue by automatically suppressing common platform domains.