It simply means you assert that you either hold the copyright yourself, or have been authorized to act for a party that does. Basically it is to prevent a third party from issuing a request without the rights holder's explicit authorization. This is largely to prevent it from being abused as a form of censorship and to prevent a rights holder from attempting to insulate themselves from the enforcement action (eg: a counter-suit). Really the concept of ownership is a rather vague status when to comes to the collective whole that is the Bukkit project and it's not clear what, if any, rights that would actually constitute. The only thing that is clear, based on public knowledge, is that there was no CLA and thus each contributer holds the copyrights to their contributions (except possibly those hired by Mojang /may/ have transferred those to Mojang or another entity).