It is the version of rust that is under active development, and also contains experimental features that aren’t in the “stable” rust compiler.

Rust has “nightly” and “stable” toolchain variants called “release channels”. The “nightly” version is literally just the compiler and all the other tools built in the CI/CD pipeline on a nightly basis with the latest branch including various features that need further development before they go to stable.

The process and reasoning are described here https://doc.rust-lang.org/book/appendix-07-nightly-rust.html