Spelled out, TurboFan and IonMonkey are JITs that take longer to run but can do more optimizations. The idea is to run them on code that executes a _lot_ of times, so the faster code generated pays for the extra JIT time. If you wrote a game in straight JS and it contains some code that runs for a bunch of in-game objects every frame, that's the kind of thing these JITs could really help with.

I'd compare IonMonkey in SM to Crankshaft in V8. Ion was actually started in response to Crankshaft.

TurboFan is a new even-higher-tier, even-more-expensive-to-compile, but potentialy even-more-optimized compiler backend the V8 team is working on.