Hi. The doppler effect is compression or expansion of waves due to something moving towards you or away from you. You can hear it most easily in sound waves, where the compression results in a higher pitched sound coming toward you, and expansion means a lower pitched sound going away from you.
Brilliant question! The Doppler effect is the change that you notice in the frequency of a wave when the source of that wave is moving relative to you! Have you seen this video – Charlie explains it pretty well!! 🙂 🙂