It's always good to have an example when you are explaining a new concept.
Where is this explanation going? I have no idea, but the comic is titled Doppler Effect, which, according to Wikipedia is "the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave." That sounds like gibberish to many of us, but with an example, it can become clearer. The siren is the most common example, but what it has to do with the appearance of more distant galaxies is anyone's guess. This teacher is suffering from another effect, that when you have a captive audience, you are always tempted to go off on an interesting tangent. Strangely, I feel dumber for having attended this class. This comic is from Randall Munroe at xkcd.