Okay kiddo, let me explain what a diocese of Moesia is.
Think of it like a big group of churches that are all located in the same area. A diocese is a term used in the Christian religion and it is a way of organizing churches.
Moesia, on the other hand, was an ancient region located in what is now modern-day Bulgaria, Serbia, and Romania.
So, a diocese of Moesia is a group of churches located in that region. Each diocese has a bishop (kind of like a leader) who helps guide and manage the churches.
This term is not really used anymore, but it was important back when the region was under the control of the Roman Empire and the Christian religion was just starting to spread.