What's JIT? I hope the following

First, we need to make clear what is JIT, and we will completely know about that when done some exercises.

1.The full name of JIT is Just In Time, it means program could be working meanwhile compiling next code. In other words, only needs to be compiled the header of whole program first and the rest  of program can be compiled later. So it will greatly reduce compilation time of the whole program, at the same time it improve the efficiency of the whole program.