An idiom is a word or phrase with a culturally understood meaning that is different from what it suggests. If you want to speak English like a native you cannot avoid learning some popular English idioms.
For example: "A piece of cake", this means something was easy. So you could say the journey to the airport was a piece of cake.
So can you guess what this one means and how you would use it?
" To let the cat out of the bag"