The amount of shifting (red or blue) depends on the speed between the light source and Earth.

The statement you provided is referring to the Doppler Effect, specifically as it applies to light (also known as redshift and blueshift). Here's a step-by-step explanation:

1. The Doppler Effect is a phenomenon that occurs when the frequency of a wave changes for an observer who is moving relative to the wave source. This effect is commonly observed with sound waves (e.g., the change in pitch of a passing siren), but it also affects light waves.

2. When a light source is moving away from an observer, the light appears more red than it actually is. This is known as redshift. The reason for this is that the motion of the source causes the light waves to "stretch out," which increases their wavelength and shifts them towards the red end of the light spectrum.

3. Conversely, when a light source is moving towards an observer, the light appears more blue. This is known as blueshift. In this case, the motion of the source causes the light waves to "compress," which decreases their wavelength and shifts them towards the blue end of the spectrum.

4. The amount of redshift or blueshift (i.e., the change in observed color) depends on the relative speed between the light source and the observer. The faster the source is moving, the more pronounced the shift will be.

5. In the context of astronomy, redshift and blueshift are used to determine the speed and direction of distant galaxies. If a galaxy's light is redshifted, it means the galaxy is moving away from us. If it's blueshifted, the galaxy is moving towards us. The amount of shift can also give us an idea of how fast the galaxy is moving.