The disk type refers to the amount of room between the hub and the spokes of the wheel.
Low disk means that there is less clearance between the hub and the spokes, in relationship to the offset of the wheel. Basically, a low disk face allows for the use of a larger outer lip than a standard face.
Here is a picture I found:
For these wheels, "standard disk" refers approximately to "MD" in the picture, and "low disk" is more like "SL".
It's kind of like a built-in spacer for brake clearance. I hope that helps!