Before Einstein entered the stage of modern physics, Newton's laws reigned. Newtonian physics provided an extremely accurate model for normal observable occurances. For example, take the following scenario. John travelling forward on a flatcar at 10 mph. He throws a ball in the same direction at a speed of 10 mph. To his friend Sally, who is standing at the station, the ball seems to move at 10 mph + 10 mph, or 20 mph. According to Newtonian physics, one simply had to add the velocities of John and the ball relative to John to find the velocity relative to an observer.
This led Einstein to ponder a similiar situation. Replace the flatcar with a spaceship capable of travel near the speed of light. Now, instead of throwing a ball, John turns on a flashlight. If he were to measure it, the light would be escaping him at a rate of about 3 x 108, otherwise known simply as "c." If John were travelling at half the speed of light, or .5c, according to Newton Sally would measure the light as travelling 1.5c, faster than the speed of light.
Contrary to Newton's laws, experiments indicate that both John and Sally would record the light as travelling at the SAME speed!
Therefore Einstein reasoned that because the speed of light is a constant, there may be an adjustment in time itself. This has led to the following concept.
Two mirrors are placed 6 inches apart with the reflecting sides facing one another. A single photon (or light "packet") is placed between them so that it oscillates by bouncing between the mirrors. Each oscillation is said to be one "click"