Centering is basically coming back to yourself, in a sense. It is focusing on a point within your body, some say near the solar plexus (some belief systems say that is the place where your energy is stored). It can be used to visualize gathering your energetic resources either before a work, or afterwards.
It is often paired with grounding, which is visualizing an energy connection to the earth (and heavens in some cases) either through your center, or from your base (if standing, from your feet. If sitting, well, you get the idea). It is to help feel rooted, to peacefully release excess energy, or to gather energy as needed.
There are a lot of methods listed online, and some may be more directly related to a given path.