What is a CDS annotation?

A Core Data Services (CDS) annotation is an enabler where you can add SAP ABAP and component-specific metadata to the source code of any Core Data Services entity. 

There is a mandatory annotation that needs to be included in a Core Data Service. It is @AbapCatalog.sqlViewName: There are a lot of annotations of the Data Definition Language (DDL) such as AbapCatalog, Access Control and ClientDependent.