glibc is the library that provides basic functionality for C programs. It provides the bottom level implementation for things like opening files, requesting memory, and other OS-level stuff.

glibc isn't the only implementation out there. Even on Linux, there are other options, such as muslc.

It gets updated regularly, as the C standard or operating system needs. So while it has been around for a very long time (by software standards anyway) it's still an active and evolving piece of software. --and one that underpins many critical functions of our systems.