Idioms are phrases that do not make literal sense but have understood figurative meanings. An idiom is an expression whose meaning cannot be understood from the individual words but has a meaning implied by custom. Examples given include common idioms like "it's raining cats and dogs" which does not literally mean animals are falling from the sky, and "break the ice" or "get the ball rolling" which refer to initiating an interaction or activity. The document discusses idioms related to food, numbers, and animals as well as what it means to "feel like a fish out of water."