It's pretty clear to me.

>> JIT, or “Just in Time” is a compilation design that implies that compilation happens on demand when the code is run the first time.

>> What people tend to mean when they say a JIT compiler, is a compiler that emits machine code.

A JIT compiler is a compiler that emits machine code the first time that code is run, vs an AOT compiler which emits machine code when the code is built.