The spud is the metal bit that goes on an opening in, for instance, the hole in the bottom of a sink, to provide an attachment point for the drain line. A spud wrench tightens the nut that fixes this fitting in place. Why Ridgid needed to make a special wrench for something that can be done with a crescent-type adjustable wrench, I'm not sure; maybe the Ridgid design predates the common availability of crescent wrenches.