JIT means "just in time". I'm not sure exactly how it works, but I think it's a method of queuing and processing stuff better, so that when the end result is happening, it is being done more smoothly.
IIRC, the concept of JIT came originally from the Java world, and the same sort of idea was thought beneficial in emulation as well.
AFAIK, it's like emulating a CPU with a 8 meg L1 cache, instead of one without cache at all (which, IIRC, is the "normal" way of emulating). This is said in extremely simplified terms, and I'm no coder, but that's how it was explained to me... :-D
