Idioms are expressions that mean more than the sum of individual words. They are learnt so that you will understand what is being said. If you want to understand a language, you know what the idioms in that language mean. Figure out its meaning.
For example,
"You need to very brave to be a superhero,"
he said to her.
If you guess the meaning of the idiom from individual words, you would think that she should be very brave in order to be a superhero.
There are about 15,000 English idioms but some are particular to certain regions and countries. It's useful to know what they mean because you will see them in the books you read. It's easy to guess the meanings of idioms from the context so you would have to memorise the meanings and look out for the idioms.
