it means that the frame in the rear above the axle has been raised or notched to allow the rear axle to travel further which will allow it to set lower.
I'm guessing your talking about c-notched,which is where the frame ,on a lowered car, is cut to allow suspension travel clearance for the rearend of the car.
Travisty, It's as the guys said, increasing the axle to frame clearance so the car or truck can be lower without the frame hitting the axle. We used to just put a half circle cut from 3-1/2 or 4 inch tube in the frame and then reinforce the frame to give a bit more clearance but now they use the big notches to get enough clearance for the rig to lay out on the ground when you let the air out of the bags.