What an idiom is

Meaning that is not compositional.

The definition

An idiom is a fixed phrase whose meaning cannot be derived from its parts. Kick the bucket is perfectly grammatical and gives no hint that it means to die.

Fixedness matters: kick the pail means nothing.
Collocations are related: strong coffee rather than powerful coffee.

Why they exist

Idioms compress a whole situation into a few words and signal shared background. That efficiency is also what makes them exclusionary.