Saying your house is "neat as a pin," or that running a marathon is "a piece of cake" and examples of idioms.
Idioms are words that are not to be taken seriously (most of the times) and can also be referred to as overstatements. They are mostly used by people who speak English. Idioms can be used for many things, and you've probably heard somebody say "it's raining cats and dogs outside" at least once in your life. That idiom is just another saying for "it's raining pretty hard outside."