I'm sorry but the landing page at fuzix.org (the top page nonetheless) is terrible as it does not even try to explain what FUZIX even IS. I went to the GitHub project page, which contains some more details, but it still doesn't answer the question and only talks about how FUZIX differs from UZI.
To be honest, I still have no idea what I'm looking at.
Take a guess. The first two words of the web page is "Fuzix" and "OS", it can't be that hard.
>it does not even try to explain what FUZIX even IS
Fuzix is a very simple UNIX clone (and a for of UZI) started by Alan Cox, an ex-Linux kernel developer) as his retirement project.
It aims to run on old CPUs like Z80 and on microcontrollers. I found it when searching what OS can I run on the $4 Raspberry Pi Pico.