[Image source] What exactly is “redshift”?  Redshift is defined as: 

[Image source] What exactly is “redshift”?  Redshift is defined as:  a shift toward longer wavelengths of the spectral lines emitted by a celestial object  that is caused by the object moving away from the earth. If you can understand that, great! But for those of us who cannot, consider the celestial bodies which make up our night sky. Did you think they were still, adamant, everlasting constants? They may seem to stick around forever, but… Boy, you were wrong. I’ll have you know that stars are born and, at some point, they die. They move, they change. Have you heard about variable stars? Stars undergo changes, sometimes in their luminosity. (We are, indeed, made of the same stuff as stars). So, stars move. All celestial bodies do, actually. You might have heard about some mysterious, elusive thing called dark energy. Dark energy is thought to be the force that causes the universe to expand at a growing rate. If it is proven to exist, dark energy will be able to explain why redshift occurs. Maybe you can understand redshift by studying a visual: [Image source] These are spectral lines from an object. What do you notice is different in the unshifted, “normal” emission lines from the redshifted and blueshifted lines? The redshifted line is observed as if everything is “shifted” a bit to the right– towards the red end of the spectrum; whereas the blueshifted line is moved to the left towards the bluer end of the spectrum. Imagine if you were standing here on earth and some many lightyears away, a hypothetical “alien” was standing on their planet. With this image in mind, consider a galaxy in between the two of you that is moving towards the alien. You would then observe redshift (stretched out wavelength) and the alien would observe blueshift (shortened wavelength).  Here, Symmetry Magazine explains redshift in their “Explain it in 60 seconds” series.  A simple, everyday example of this concept can be observed if you stand in front of a road. As a car (one without a silencer) drives by, the pitch you observe changes. This is known as the Doppler effect. Watch this quick youtube video titled “Example of Dopper Shift using car horn”:  (You may not be able to view it from the dashboard, only by opening this post on the actual blog page. You can watch the video by clicking this link).  Notice how as the car drives past the camera man, the sound changes drastically. Understanding redshift is important to scientists, especially astronomers and astrophysicists. They must account for this observable difference to make the right conclusions. Redshift is one the concepts which helped scientists determine that celestial bodies are actually moving further away from us at an accelerating rate. -- source link

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