Dressing an electrode means mechanically machining or forming the face of the worn electrode back to its original geometry. All resistance welding electrodes wear during use. In the case of spot welding this appears in the form of what is referred to as mushrooming. The weld face of the electrode begins to spread out perpendicular to the length of the electrode.
When this face grows it leads to discrepant welds due to inadequate current for the increased weld face size and in ability to contain the weld nugget. “Dressing” the electrode by machining the weld face back to its original face configuration before you experience weld failure is the accepted method. People can use automated pneumatic dressing stations with robots, hand held pneumatic tools, hand mechanical tools, emery paper or files. Files are not recommended since they are not repeatable or controlled. Removal of the part to a lathe is the most controlled method. The goal is to return the face back to its original dimensions.
Normally the buildup on weld face is not machined/removed, just the excess around it. This is especially helpful when welding galvanized to give you a smooth start up when returning the electrode to service.
Reference: RWMA Resistance Welding Manual 4th Edition