Basically, it refers to the space between the mounting pad and the back surface of the rim face. A disk(Standard disk) will have a relatively large space, compared to the O disk, allowing for the use of large calipers. Big brake friendly. O disk(low disk) will have minimal brake clearance. The space from the mounting pad and back of the rim will be slim compared to the standard/high disk types. O disk will also allow for a larger lip for the rim.
Someone correct me if I'm wrong.
Just in case I didn't make any sense:
http://youtu.be/733NtHC01mQ