The web part can be used to create a newsletter unsubscription page with a custom design that will be used instead of the default system page. The URL of such a page can be assigned to individual newsletters through their Unsubscription page URL property, or globally in Site Manager -> Settings -> On-line marketing -> Newsletters -> Newsletter unsubscription page URL. Once assigned, the unsubscription link for the given newsletters will lead to the page. There, the web part will automatically process the query string parameters and perform the required unsubscription.
Help URL: http://devnet.kentico.com/docs/webparts/index.html?NewsletterUnsubscription_overview.htm