Idiomatic expressions are hard to understand literally by definition!
An idiomatic expression is a phrase whose meaning cannot be understood just by looking at the words in a dictionary.
Idiomatic expressions are colloquial (not a simple word, I know) - this means that they only have meaning to one specific culture. In even simpler terms - they are expressions that only a native speaker of the language understands, or someone who lives in that place.
Here are a few examples -
Break a leg!
This means good luck! But if you just read the words - break. a. leg. there is no way you'd ever know that that means good luck.
It's a piece of cake!
This means it is really easy. But if you just think about what a piece of cakes means, you would never know that. Cake is delicious, but doesn't have anything to do with being easy!