An idiom is an expression that is characteristic of a particular language, but that means something different from the literal meaning of the words. For example, the idiom "having the upper hand," has nothing to do with hands. It means to have the advantage in a situation.
People frequently use idioms because they make speech and writing more colorful and interesting. Here are some commonly used idioms and their meanings.
Sell someone short - underestimate someone.
Sitting pretty - be in a fortunate position.
Hit the ceiling - become very angry.
Pull someone's leg - fool someone.
Wet blanket - a dull or boring person who spoils the happiness of others.
Keep under one's hat - keep something a secret.
Get off someone's back - stop bothering someone.
Shape up or ship out - behave properly or leave.
Make ends meet - pay one's bills.
Out of the woods - out of danger.
In stitches - laugh very hard.
Spill the beans - reveal a secret.
Tongue in cheek - not serious.
For the birds - uninteresting and meaningless.
Shake a leg - hurry.
There are hundreds, even thousands of idioms that are used in the English language. You may find it helpful to purchase one of the many dictionaries of idioms that are available.
Here is one more idiom.
Give it your best shot - try hard.
Idioms are hard to learn, but give it your best shot.