Preaching to the choir is a common English Idiom that means “trying to convince someone of something that they already believe.” Imagine a preacher in a church, directing his sermon at the choir. They already believe it; that’s why they’re in the church choir. Preaching to the choir doesn’t change anyone’s mind.
“Preaching to the choir” sometimes implies that speaker has chosen the wrong audience, either by mistake or on purpose, because an audience who already agrees with you is easier to deal with. “Preaching to the choir” can also just mean, “I really agree with you!”
“The animal-rights activist spoke at vegetarian organizations all over the country. He hoped he was making some converts, but he was probably just preaching to the choir.”
A teacher is upset because so many students have skipped class, and lectures the remaining students about the importance of attendance; she’s preaching to the choir because everyone who really needs to hear her lecture is gone.
If you were to tell me that the internet is a great place to learn languages, I could tell you, “You’re preaching to the choir!“