This is a special processor that allows a processor or sub-tree of processors to be pre-calculated and it's result stored. This is useful if you have certain calculations that are repeaded multiple times.
You can define the Fragments in the fragments section of a resource, then us the fragment processor to insert the pre-calculated result into your node-tree.
key
All fragments inthe fragments section of your resource are pre-calculated every time that the endpoint is called. That result value is then inserted as the result of your fragment processor without having to re-calculate it.