What is a phrase or expression whose meaning cannot be understood from the ordinary meanings of words in it?

A phrase or expression whose meaning cannot be understood from the ordinary meanings of words in it is called an idiom.

Here are some examples:

* "Kick the bucket" means "to die"

* "Spill the beans" means "to reveal a secret"

* "Break a leg" means "good luck"

Idioms are a common part of language and can be tricky for non-native speakers to understand. They are often used figuratively and have a meaning that is different from the literal meaning of the words.