A suppression list is an email list of email addresses that you do not want to send to.
Normally an advertiser will provide you a list of emails they do not want to mail to and in this case you upload it as a suppression list inside the system.
Then when you create a campaign in addition to selecting your email list you would also select that suppression list.
In additon to normal text file with a list of emails we also support, MD5, SHA56, and a list of domains as suppresion lists.