What does CMS mean in relation to Java's Garbage Collector?

Concurrent Mark Sweep

(Or as Tom mentions in the comments, mostly Concurrent Mark Sweep.)

It's a method or algorithm for garbage collection that, in theory, affects the performance of the application less than the older methods.

The concurrent mark sweep collector, also known as the concurrent collector or CMS, is targeted at applications that are sensitive to garbage collection pauses. It performs most garbage collection activity concurrently, i.e., while the application threads are running, to keep garbage collection-induced pauses short

G1 is the next level being considered/developed.