An Idiom
An idiom is a common phrase or expression with a figurative meaning. You cannot understand it just by looking at the literal definitions of the individual words; instead, the phrase must be learned and understood as a whole.
Every spoken language in the world uses idioms. While the specific expressions vary drastically between cultures, the use of figurative, non-literal language is universal across all linguistic families.

A blessing in diguise
A good thing that seemed bad at first

Beat around the bush
Avoid saying what you mean, usually because it is uncomfortable

Better late than never
Better to arrive late than not to come at all
Bite the bullet
To get something over with because it is inevitable

Break a leg
Good luck
Call it a day
Stop working on something
Everyday Life & Hard Work
Burn the midnight oil: Staying up very late into the night working or studying intensely.
Cut corners: Doing something poorly, cheaply or skipping steps to save time or money.
Go back to the drawing board: Starting a project or plan over from the very beginning because the first attempt failed.
Hit the sack: Going to sleep or heading to bed for the night.
Wrap your head around something: Managing to understand a complicated, strange or difficult concept.
Emotions, Health & Well-bein
On cloud nine: Feeling extremely happy, excited or blissful.
On thin ice: Being in a highly risky, dangerous or precarious situation where a mistake will lead to trouble.
Pull yourself together: Calming down, managing your emotions and regaining your composure.
Under the weather: Feeling slightly sick, ill, tired or unwell.
Communication & Relationships
Barking up the wrong tree: Accusing the wrong person, making an incorrect assumption or looking for solutions in the completely wrong place.
Break the ice: Initiating a conversation or doing something to relieve tension and make people feel comfortable in a social setting.
Give someone the cold shoulder: Intentionally ignoring someone, acting unfriendly or treating them with coldness.
Spill the beans: Accidentally or intentionally revealing secret or confidential information.
The elephant in the room: A massive, obvious problem or controversial issue that everyone knows about but no one wants to talk about openly.
Money & Value
Two (or ten) a penny: Something that is incredibly common, abundant and therefore holds very little value.
Break the bank: Something that is exceptionally expensive or costs far more money than you can actually afford.
Costs an arm and a leg: An expression used to describe an item that is wildly overpriced or expensive.
From rags to riches: Going from extreme poverty to immense wealth and success.