What is a "center lock" wheel?
 

http://www.bikemannetwork.com/biking...V/MAV-32365010

What does center lock refer to?

The disc brake mounting method. So for this hub you would need center lock disc rotors.

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.

It refers to one of two disc-to-hub mounting interfaces:

1) "Centerlock" is Shimano's interface.

2) "Six-Bolt" is everyone else's interface.

Note, not all shimano hubs are "Centerlock". "Centerlock" is not compatible with "Six Bolt"; however, you can get an adapter to turn a "Centerlock" into a "Six Bolt".

In general, there are more aftermarket parts/accessories for "Six Bolt" pattern.