An idiom is an expression with a different meaning than its literal one, e.g. if something is hard to find you may say it's like trying to find a needle in a haystack.
Idioms are fascinating, but they don't always make much sense until someone explains them. It can be useful in some types of writing to use idioms, especially when you are writing dialogue for a character or perhaps an advertisement or travel brochure. In other types of writing, especially the formal, it is best to avoid idiom. These phrases are so well known that your writing might sound clichéd.
So remember, the grass is always greener on the other side, no news is good news, you can't have your cake and eat it, and always let sleeping dogs lie!
Put your best foot forward and have a go at this quiz all about idioms!