Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.
Idiom:
have an ace up one's sleeve
Meaning:
to have something that you can use to gain an advantage (in a card game the ace is often the most valuable card and a cheater could have an ace up his or her sleeve to use against an opponent)
Example:
I have an ace up my sleeve which should help me when I meet my boss tomorrow.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.