Okay kiddo, so you know how in school you have a principal who helps make sure everything is running smoothly and people are following the rules? Well, the Roman Catholic Church also has leaders who help make sure things are running smoothly and people are following the church's teachings.
Now, a diocese is like a big family of churches that all have the same leader. The Roman Catholic Diocese of Pesto is one of these families, but it's located in a certain area of Italy called Pesto.
The Bishop of the Roman Catholic Diocese of Pesto is the leader of all the churches in that area. He helps the priests and other leaders in the churches make decisions and support their communities.
Basically, the Roman Catholic Diocese of Pesto is a part of the bigger Roman Catholic Church and is led by a special person who helps the people in that area follow their religion.