Its an alternative libc instead of gnu.

I've generally seen it for embedded systems due to smaller size and you can statically link it.

But there are some compatibility drawbacks for non musl binaries as well as source that intentionally or unintentionally relies on glibc behavior or non standard functionality.

You don't want to mix and match your libc's on a running system.

Buildroot may be a good way to start to play with musl (or uClibc, another small c library).

I found building musl easier than uClibc, Xephyr, newlib, or (especially) glibc. Glibc is an Ouroboros of hacks at build time.

Also a good example of what happens when trying to write portable C code, even when constrained to UNIX/POSIX platforms.