Message board unsubscription
The Message board unsubscription web part is a special web part that handles message board notification unsubscription reuqests. Notification e-mails contian unsubscription links leading to a page where this web part is placed, with the subscription specified in URL querystring. By clicking the link, users get redirected to the page where this web part performs their unsubscription so that they no longer receive the notifications.
The web part belongs to the Message boards module. Full documentation of the module can be found in Kentico CMS Developer's Guide -> Modules -> Message boards.