https://www.phoronix.com/news/Mesa-24.3-Juiciest-Refactor
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
Mesa is more than just the fallback, it's literally the entire userland graphics stack for all the open-source drivers. Intel, AMD, it's also where the Apple Silicon drivers for Asahi lives and more.
It's not slow at all, unless you end up with the CPU renderer (llvmpipe/lavapipe) due to an unsupported graphics card. That's usually NVIDIA users because nouveau isn't great. But even then, when NVK comes out, it's also in mesa.