It literally means it has weights in the center, which are not used on cheap production or moderately priced cranks. Look at pictures of Mustang Gt (newer) cranks, some of them are center weighted.
A crank without the center weights has to use weight way out of line from the rods and positions in the center, which takes away the vibrations, but the bearings on the 2nd, 3rd and 4th mains have to take extra loads, and it puts stress on the crank,flexing it. The block has to transmit that balance force also,putting stress on it. Not too big a deal at low rpm, but as rpms go up, the loads go WAY up.
