Redshift is a natural phenomenon observed by astronomers observing distant galaxies. The light from sources moving at significant speed away from the observer appear to be lower in frequency. The light from sources moving at a significant speed towards the observer appear to be higher in frequency, which would be called blueshift if it was commonly observed. Red and blue shifts are a consequence of the Doppler effect on light waves.
These shifts in frequency are easily measured because the emmision and absorbtion patterns for molecules are distinctive. The amount of frequency shift can provide a means of measuring the relative speed between the source and the observer.
Edwin Hubble was one of the primary researchers in this area of cosmology. It has been observed that the further away a galaxy is from us, the higher the relative speed away from us (greater redshift). Looking back in time, this suggests that all the matter in the universe originated from the same point. This has lead to the Big Bang theory.