In a nutshell, figuring out whether a variable or object is going to be entirely confined to a single thread and scope and using that information to perform optimizations (like sticking things into registers) or avoid unnecessary locking.
afair, the most important optimization there is that something can be allocated on the stack instead of the heap, which is quite an optimization. EDIT: it does not actually have anything to do what you can stick in registers--at least nothing comes to my mind w/o thinking too hard about it...
relevant papers directly from the hotspot-jvm are:
"The Java language does not offer any way to explicitly allocate an object on the stack, but this fact doesn't prevent JVMs from still using stack allocation where appropriate. JVMs can use a technique called escape analysis, by which they can tell that certain objects remain confined to a single thread for their entire lifetime, and that lifetime is bounded by the lifetime of a given stack frame. Such objects can be safely allocated on the stack instead of the heap. Even better, for small objects, the JVM can optimize away the allocation entirely and simply hoist the object's fields into registers."
