What is meant by the doppler effect?

The doppler effect is the change in frequency and wavelength of waves coming from a wave source moving relative to you. If a wavesource is stationary then the waves in all directions all have the same frequency and wavelength, with the waves forming concentric (surrounding) circles around the wave source. If a wave source is moving, however, the waves located in the direction it is moving will be compressed, leading to them having a shorter wavelength and a higher frequency. On the other side of the wave source, the opposite occurs. You may have heard this effect from an ambulance, as the pitch of it's siren rises when its frequency increases (when it approaches you) and lowers when it moves away. The same principle can be used to determine the velocity of distant stars.