From the Microsoft Exchange Team blog:
The release includes fixes for customer reported issues, minor product enhancements and previously released security bulletins, including MS15-103.
Cumulative Update 10 does not include updates to Active Directory Schema, but does include additional RBAC definitions requiring PrepareAD to be executed prior to upgrading any servers to CU10. PrepareAD will run automatically during the first server upgrade if Setup detects this is required and the logged on user has sufficient permission.
The updates released today are important pre-requisites for customers with existing Exchange deployments who will deploy Exchange Server 2016.Cumulative Update 10 is the minimum version of Exchange Server 2013 which will co-exist with Exchange Server 2016.
For the full list of fixes check: KB3078678
Cumulative Update 10 is available for download here.