What’s an Idiom?
Broadly speaking, an idiom is a widely used phrase that, when taken as a whole, has a particular meaning that you would not be able to deduce from the meanings of the individual words. The ubiquitous greeting “How are you doing today?” is an example of an idiom. Normally, how means “in what manner” or “to what degree.” Taken literally, the question doesn’t make a lot of sense. But fluent English speakers understand the idiomatic meaning; “How are you doing today?” usually just means “hello.”