An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
knock (some) heads together
Meaning:
to scold some people
Example:
The coach decided to knock some heads together in order to improve the team.
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.