https://blog.racket-lang.org/2019/01/racket-on-chez-status.h... and the links at the top should provide the explanation. This paper: https://www.cs.utah.edu/plt/publications/icfp19-rddkmstz.pdf has more details.

Basically, Chez is a now-open-source runtime that's very close to what Racket needs, while also being written mostly in Scheme instead of in C (as the original Racket runtime & compiler is). Building on Chez allows us to cut the amount of C by a factor of ~10 while getting a better architecture.

Chez is AOT compiled down to the metal; it's not an interpreted VM or a JIT compiler. My initial reaction was "Oh good. This means Racket will run much faster!" But that's not really true. Some things are faster; some aren't. The real reason is that because Chez compiles to the metal, it makes maintainability easier because less C code is needed.