"Preaching to the choir" is an idiom which means sharing your ideas with people who already agree with those ideas. It is considered to be a bit of a waste of time.
You can imagine that this phrase comes from the idea of a minister preaching to a church choir (which is a group of singers). Since the people in the church choir are already involved in the church, they probably already agree with what the preacher is saying.
When someone tries hard to convince you of something that you already believe, you can tell them that you already agree by saying "You're preaching to the choir."