I think it's new name for MRU (most recently used) as it was called in XP.
It's like your History Cache/most recently viewed, in your browser.
The JumpListCache is presumably where W7 remembers/makes a note of, what you have used/pinned etc to your JumpList so that it can display them. It also has more features than the old MRU.
How to Change the Number of Recent Items to Display in Windows 7 Jump Lists ClickHere