What is an Idiom?

An idiom is a phrase or sentence in which the meaning of the whole sentence is different from what you would expect. The words are not literally true. For example:

To paddle your own canoe doesn't mean that you are really in a canoe. It means that you can do things for yourself.

There are many idioms! Two further examples are:

To let the cat out of the bag which means to give away a secret.

To bury the hatchet which means to make peace.