Imagine a wave source is moving relative to an observer. The wavelength and frequency of the waves detected by the observer will have changed (shifted) from the original produced by the source. This is called the Doppler Effect.
When the source moves away from the observer, the observed wavelength increases and the frequency decreases.
When the source moves towards the observer, the observed wavelength decreases and the frequency increases.
The Doppler effect can be demonstrated with sound waves.
Galaxies are large collections of stars. Light observed from distant galaxies has been 'shifted' towards the red end of the spectrum. This is known as the red-shift and means that the frequency has decreased and the wavelength increased.
A blue-shift would indicate that a galaxy is moving towards us. We are able to see these effects by observing dark lines in the spectra from galaxies.
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