What is JIT trying to do?

[vish-pr]

This is a very basic question, and reading code should clear it, I have tried it and I think I understand it some kind of caching on request and response of functions. But still I am not sure,

1. What is problem statement it is solving?
2. How can we cache responses, if it will be different for different for input?

[wozeparrot]

1. Speed
2. It doesn't cache responses, it caches kernels then just runs them without going thru the expensive complication process again.