Okay kiddo, so you know how sometimes grown-ups get married and promise to love and take care of each other forever? Well, some grown-ups who work for the church also want to get married and promise to love and take care of each other forever, but there are some rules about it.
The rule used to be that if you wanted to work for the church, you couldn't get married. But then, about 900 years ago, someone decided that maybe it was okay for priests and other church workers to get married as long as they did it before they became a priest or worker for the church.
Nowadays, in most churches, people can become priests or other workers even if they are already married. But there are some churches that still only allow priests who are not married, and there are some rules about what kind of people they can marry if they do want to get married.
Does that make sense, kiddo?