Breathing
When a person breathes in, they bring air into their lungs.  Air has oxygen in it.  The oxygen goes from the lungs into the person's bloodstream.  When oxygen goes into the bloodstream, extra carbon dioxide comes out and goes into the lungs.  This is called gas exchange: basically, oxygen and carbon dioxide are changing places.  Oxygen is now in the bloodstream, which can carry that oxygen around to every part of the body.  Also, carbon dioxide is now in the lungs, where it can be breathed out.