Lorentz factor
The speed of light, c, is 670,616,629 mph. So if you are in a car traveling half the speed of light (0.5c) and you turn on your headlights, the light moves away from you at 1 c... or is it 1.5 c? It ends up that c is c no matter what. The next section explains why it's not c - 0.5c.