An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
keep (someone) company
Meaning:
to sit and stay with someone (especially someone who is lonely or sick)
Example:
I stayed home last night so that I could keep my mother company.
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.