The sternum in relation to the rib cage, via Wikimedia Commons
The sternum is a large flat bone which runs down the middle of the chest over the top seven ribs. It's attached by cartilage to the ribcage and the clavicle. It's positioned to protect the heart, lungs, and major blood vessels from major trauma.
Unfortunately, the sternum is often in the way when doing surgery in the thoracic cavity, and occasionally has to be cut open through the middle.
Fractures of the sternum are very uncommon. The most usual cause is impact with the steering wheel of an automobile in a collision. Another common cause is repeated trauma playing sports like gridiron football and rugby.