It is an absolutely revolutionary OS by some of the original creators of Unix, that extends its core concepts in more coherent and elegant ways into the world of modern computing, instead of having everything from networking on up be tacked on by people who were perfectly capable but lacked the vision.
Examples:
Instead of NAT, if one machine on your network has the internet and the others don’t, you can say “use that other machine’s network stack now” and boom everything works. Your machine knows what its real external IP address is, it can listen on world-facing ports on the other machine as it needs to, everything works and is simple.
There’s a command for “run the rest of this session’s commands on that other machine’s CPU / memory” and it all just works. The sensation is that your computer just got magically faster.
Etc etc. I actually haven’t played with it extensively, and deployment is so limited that I’m not sure how useful it would even be, but if you are a fan of well made OSs that do things in a genuinely different fashion, it is objectively the best option to play around with. sdf.org has a place you can get an account on their Plan 9 machines and they do little free beginner courses in it over livestream.