Mesa is the cross-platform, open source implementation of the OpenGL, Vulcan and OpenCL protocols for use in creating graphics on screen.

It’s the default GL library on Linux for when you want to write code that creates 3D graphics.

You can juxtapose it to Metal on Mac’s or DirectX 3D on Windows.

It’s an open source linux graphics driver.

It is very widespread, despite being quite slow, because it works. It ships by default with almost everything, and is the fallback when card-specific drivers fail

Edit: what Max_P said