Remacs is a rewrite of Emacs' C code within the Mozilla-sponsored programming language. In terms of why there is this separate Rust-based implementation of Emacs, "Porting to Rust gives us lots of opportunities. We can leverage the rapidly-growing crate ecosystem. We can drop support legacy compilers and platforms (looking at you, MS-DOS). We can add docstrings and unit tests to core functions that aren’t exposed to elisp. It’s also a ton of fun. Remacs is based on Emacs 25.2."
Those wishing to find out more about this Rust-based Emacs implementation, Remacs, can read this blog post for all the details.