It refers to a Shimano system of locking you into only using Shimano's disc rotors.

Sorry for the smarmy comment, but I do have my doubts as to what question centerlock is an answer to. As with almost anything Shimano makes, it does work fine, but later on the splined interface that the rotor slides onto can develop slop, and there's no fix for that other than replacing the hub, the rotor or both. Standard 6-bolt rotors work just fine, and are my personal preference for maximum choices in aftermarket hardware.