Subscriptions allow users to receive an e-mail notification when a new post is added to the selected forum.
By clicking the New subscription button, you can add a new e-mail address. If you check the Send confirmation e-mail to subscriber check-box on that page, the subscriber will receive a confirmation e-mail, telling them that they have been subscribed.
More resources can be found at:
Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?subscriptions_tab2.htm