An idiom is a phrase that has a figurative meaning that is different from the literal meaning of the individual words.
For example:
* "Kick the bucket" doesn't literally mean to kick a bucket, it means to die.
* "Break a leg" doesn't mean to actually break your leg, it means good luck.
These are just a few examples, but there are countless idioms in every language. They often add color and depth to our language and can be quite fun to learn!