The doppler effect is a phenomenon that happens with sound or sound waves. It's the change in frequency of a wave.
Let's say you have something that's making noise and as that thing is coming towards you, not only does it get louder but the pitch changes and it gets sharp as it comes towards you and goes flat as it goes away. So the waves are getting compressed as they come towards you and they spread out as they're going away. Something similar happens with light and we call that the redshift.
