A quick definition of accessory use:

An accessory use is something that depends on or is related to the main use of something else. For example, a garage is an accessory use to a house. It's not the main part of the house, but it's related to it and used for a similar purpose. Other types of uses include regular use, which is something that is normal or usual, and nonconforming use, which is something that is not allowed under current rules but was allowed before the rules were put in place.