It's ancient history now, but those 'rings' were supposed to match up to the (then relatively new) 386's protection levels.

Trivia: I was part of a potential startup back then, working name "Ring Zero" as we were writing for that (what would be microcode on a mainframe) - but it didn't go anywhere. This was back when a coder (e.g. me) could keep track of what was in all the preload caches and so write substantially faster code than a compiler could.