Your lungs are the organs that allow you to breathe. In order for your lungs to be able to breathe air in and out, you need many working muscles.

Your diaphragm is the major muscle of breathing. It sits below your lungs and just above your stomach and liver. When it contracts, it pushes down further into your abdomen to allow your lungs to expand and take up more room.

Muscles in-between your ribs and muscles in your neck are also very important muscles for breathing. When your diaphragm contracts, your ribs and neck muscles act to pull your ribs up and out. All of these muscles act together to give your lungs as much room as possible to expand and let air in.
As the diaphragm relaxes it pushes up and at the same time, the other muscles drop the ribs. In response, the lungs shrink and push air out.