Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.
Idiom:
in over one's head
Meaning:
having more difficulties than one can manage
Example:
The supervisor was in over his head when he began to fire people for no reason.
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.