The Registration Handler is a special class used by several components in order to handle data storage and lookup on a per user basis. Basically, each user that registers with the Registration Handler is assigned an ID, which can be used to store and retrieve user specific data.
It is very unlikely that you will ever need to interact directly with this class. The only time this might not be true is if you are creating a custom Chunk Streamer or Chunk Manager.
If you do need more information on how to use it, please take a look at the public API (link is provided at the top of this page).