AOT (ahead of time) is your code is turned into machine code before you run it.

JIT (just in time) is your code turning into into machine code as you run it. Machine code is what your CPU directly processes, so it can be very fast.

AOT languages can use more expensive techniques to make super optimised machine code since the cost is upfront when compiling the code, not when running it. But JIT languages can have features that exploit the fact that machine code is made on the fly.

AOT programming languages: C++, C, Rust

JIT programming languages: C#, Javascript, Java

It's what people think of when they mean "compiled" in the first place: ahead of time, i.e. before you start the program, as opposed to JIT "just in time" where you compile (and re-compile) parts while other parts are already running.