Centering is to place clay or ware correctly in the center of the wheel. When centered, clay can be turned to make a circular object. It is also needed when banding, so that the lines you draw go right round and meet exactly.
Wheels are marked with very finely indented circular rings, which helps centering correctly. The second photo is taken from above so you can see the marked on rings. Read more about: Wheel (c) Hand wheel