The clutch collar is the cast piece that the clutch release (throw out) bearing mounts to. While these normally do not get damaged, a bad release bearing can damage them enough to warrant replacement. The most common reason to replace the collar is from a transmission or clutch swap. It is important to make sure you use the proper collar for your application to ensure proper clutch operation.
Changed an engine or transmission and not sure which collar you have? This collar is approximately 1 1/2" long from the base to the beginning of the bearing mating surface, or approximately 2 1/16" in overall length.