An idiom is an expression that has a special meaning that's different from what its individual words say.
Idioms are a big part of how we talk, and they can be confusing if you don't know what they mean. Let me give you an example: "kick the bucket" doesn't literally mean to kick something called a "bucket." It means to die! You might hear someone say, "My grandma kicked the bucket last week," but it's not about kicking a physical object. Idioms like this can make everyday conversations sound weird if you don't understand them.
Idioms often use metaphors or exaggerations to describe things in a more interesting way. They're usually used for informal communication, so you won't find them in formal writing or speeches. When people say "spill the beans" they're asking someone to share some information that's been kept secret. It doesn't mean literally pouring beans out of a bag.
A manner of speaking, a mode of expression peculiar to a language, person, or group of people.
