It’s a micro-Sievert. Micro means one millionth. It’s abbreviated the Greek letter “mu”, which I don’t have on my keyboard, so I use a “u” which is sort of close.
A Sievert is a unit of radiation that has to do with the absorption rate of radiation in human flesh. Some guy with the last name Sievert named it.
I talk about it more at the bottom of the page in my first couple of posts.