CDS (Common Data Structure) defines higher level data structures or types on top of COS types that are used all over the PDF spec.
Examples of such data structures that are no longer COS "primitives" (like strings and numbers) or "composites" (like arrays or dictionaries) but have already domain specific behavior are the date or rectangle object.
The CDS objects are implemented as wrappers on COS objects. Every change is reflected immediately in the implementation (COS) layer and vice versa.