The Crab Nebula is a debris cloud that formed when a star exploded. The cloud has been growing ever since the explosion happened. The dark spot labeled Pulsar is the star's core that was left over after the star exploded. The spots labeled 1 through 10 are knots in the cloud where the debris is especially dense.
If you click on the button labeled "SuperNova 1956", another picture taken in 1999 will appear. Click the button again and the 1956 picture re-appears. By flipping back and forth, you can see the cloud has grown and you can see that most of the stars in the background move as well. Notice that most of the knots in the debris cloud are appearing to move faster than the most of the stars.
By measuring the how far the knots moved away from the pulsar between 1956 and 1999, you can determine when the star exploded.
To measure the distances, drag one of the small blue circles so it's on the pulsar and drag the other small circle to one of the knots. The large red circle displays how many "pixels" the knots have moved between 1956 and 1999. You may be more used to units like miles or centimeters but as long as the units are consistent between the two images, you can use any unit you wish. Pixels are just the dots that make up the images you see on your screen.