You won't dig very far into the topic of astronomy before you hear the word "redshift", it's a fundamental observation in astronomy, and it measures the speed at which an object is moving toward or away from the observer.

The concept is easy to grasp for anyone who's stood by the side of a road and listened to an automobile horn honk as it goes by, or stood next to a railroad track and heard a locomotive horn as the engine goes past. You'll hear that the pitch of the horn is higher as the horn approaches you and drops to a lower tone as the horn moves away.

This happens because, as the horn comes toward you, the sound waves are crowded together because the horn is closer and closer to you as successive sound waves are emitted. Your ear hears close-together sound waves as a high pitch. As the horn moves away, the sound waves are stretched apart because the horn is farther away as each wave is emitted; your ear interprets that as a lower pitch.

This is called the Doppler Effect, after the Austrian physicist Christian Doppler.

The emission of light behaves in the same way. Light waves viewed from an object that is approaching the observer are shifted to a higher frequency, which the eye sees as a shift toward blue. Light waves coming from an object that is receding are shifted to a lower frequency, which the eye sees as a shift toward red.

The term "redshift" comes from this shift of frequency. You hear this term much more than "blueshift" because the area of astronomy in which the approach or retreat of objects is most important is in cosmology (the study of the Universe as a whole), and cosmologically important objects are almost always moving away from us, with the amount of the redshift indicating how fast the object is moving away.

The speed an object is moving away from us would be just an interesting factoid but for the remarkable discovery in the 1920s by Edwin Hubble (of Hubble Space Telescope fame) that the more distant a galaxy is from us, the faster it is moving away from us. In fact, this relationship is now understood to be a fundamental property of the Universe, caused precisely by the fact that the entire Universe is expanding uniformly.

What does "uniform expansion" mean? Suppose you had a map of the world printed on the surface of a balloon. As you blow up the balloon, every bit of the rubber stretches, so the distances between all of the cities gets larger. And the farther away a city is from your city, the faster that city will be moving away from your city. In fact, a city that is twice as far away will be moving away from your city twice as fast, because twice as much rubber is stretching.

Note that there is no center to this expansion. Every city sees exactly the same thing. In the United States, New York can claim to be standing still while the rest of the country heads west; Los Angeles can claim to be standing still while the rest of the country heads east. And no observation can contradict either claim. In actuality, the center of the expansion is the center of the balloon, which is not even on the map printed on the balloon's surface. The speeds the cities are moving depend on nothing other than their distances.

This is very much different than an explosion. Suppose you were riding on a piece of debris caught in an explosion used to blast rocks out of the ground. You could observe the speed and direction of other pieces of debris and calculate exactly the location of the center of the explosion. This is because the other bits of debris will not all be moving directly away from you, and their speeds will not depend so much on their distances from you as on their size. Other effects would be notable as well, such as the fact that the biggest pieces of debris tend to remain close together near the center of the explosion.