The Activator Keys screen is accessed through the Admin Tools menu. You must have administrator priviledges to access it.
Upon initial installation or whenever renewing licenses, the Activator Keys page can be used to quickly process all users at once. In Mars 8, all licenses are "Named User" licenses. In other words, each license is assigned to a specific user account. Licenses come with a specific user period in which they are active and can be renewed indefinitely. Licenses can be reassigned, but only one user is assigned to one license at a time.
Most licenses are annual and need to be renewed each year by their expiration date.