Idiomatic expressions are a type of informal English that have a meaning different from the meaning of the words in the expression.
Here's an example of an idiomatic expression.
Hold your tongue.
This idiom does not actually mean that you should stick your fingers in your mouth and grab a hold of your tongue. It means that you shouldn't talk.
People "hold their tongues" when they are in situations where they want to talk, but it would be better if they didn't. So, while their tongue is ready to do some talking, they "hold" it and don't say anything.
Every language has idioms, and they can be difficult to learn if you're not a native speaker of that language.
If English isn't your native language, the best thing that you can do is have conversations with native speakers and ask them about phrases that you don't understand. Since idioms are influenced by the culture, learning the idioms of a language can be very interesting and enlightening!