Think of it as a do-not-email list. To edit the Suppression list, go to the menu and select Subscribers -> Suppression list. An entry in the list excludes specific recipients from mailings, even if they are active in specific lists.
This entry means that no mailings will be sent to firstname.lastname@example.org.
This entry means that no mailings are sent to recipients whose e-mail address ends in or contains samplecompany.com
Comparison with public Suppression lists
There is no automatic synchronization with a public Suppression / Robinson list. Legally speaking, this is not necessary, as those lists are only maintained on a voluntary basis.