Sega Saturn, Bios.bin

IM confused to exactly what it actually is, and is it required to play?

It's the BIOS of the original console, which is basically the most basic code that runs on the system. It's required to use the emulator, and the only legal way to get it AFAIK is to dump it from your own console.

It isn't required for ALL games. Some work better without it, and some won't work at all if you don't set it up.