Welcome to Gaia! ::

OK, so you have a sourse, such as a star, emmiting light. As the sourse emmiting the light gets farther and farther away from us the light has farther and farther to travel. As it travels though, the wavelength of that light gets stretched, gets longer. As a wavelength gets longer it starts to look more and more red to us. Hence the term redshifting. We can measure the wavelength of a wave we receive and we can guess what the original wavelength was, and so from that we can calculate distances.
This only works though with really far away things.

You're welcome. ^_^ Glad I could help.

Red shifting is also light's version of the doppler effect. That thing that makes something like a siren comming towards you sound paifully high pitched when it aproaches quickly, and sound much lower in pitch as it goes quickly away.
But things in space travel so fast that they can do that to magnetic waves instead of sound waves. The effect of this is to make visible light waves from sources moving away from us apear further down in the spectrum than they would normally. red becomes infra red, blue starts to turn to yellow and ultra violet starts to become blue visible light.

A freaky thing about red shifting is that it works on all forms of megnetic frequency (of which light is only a narrow part of) if an object is moving fast enough away from you it can turn ultra liolet immisions into radio waves from your perspective.

Blue shifting works like that too, in theory if you were taveling towards something making a radio frequency at high enough speads you would instead be bombarded by gama ray frequencies instead! Anothr problem with near light speed travel would be how to deal with the suddenly deadly immisions comming from every hydrogen atom you nudged out of your path!