Your heart has four valves: the tricuspid, pulmonary, mitral and aortic valves.
Each of these valves has a tissue cusp that opens and closes every time the heart beats - in fact, that's what makes the beating sound you hear. Think of the cusps like doors. The cusps make sure blood flows in the right direction through your heart's four chambers and to the rest of your body.
In the above drawing, you see the inside of a healthy heart. The blue arrow shows the direction of oxygen-poor blood flowing through the heart to the lungs. The red arrow shows the direction of the oxygen-rich blood flowing from the lungs into the heart and then out to the rest of the body.
A normal, healthy heart valve lets blood flow smoothly and freely in one direction. It closes completely and doesn't allow any blood to flow back through the valve.