AKA Ball Bashing.
Well, for Doris actually - hardly a dummy, but here we go.
Imagine I am 100 miles away from you and we are both standing completely still.
Because its Christmas, I am utilising my magic, umbrella style, multidirectional tree bauble gun, and its shooting out lovely Christmas tree baubles in every direction. No problem, they're quite light and shatterproof.
This grand emission happens once every minute. Because I am superwoman and I said so.
They are all travelling at exactly 100 miles an hour, so, barring obstructions, the first of my luvverly glittery tree balls zooming out in your direction, is going to land in your lap exactly one hour after I shot it at you.
Thats because it will need the hour to travel the 100 miles between us.
Because we both remain still, the next ball will take just as long, leaves me a minute after the first one and so gets to you exactly a minute after the first one lands. And so on.
Fine so far. In a formula, the frequency with which you get hit by my yuletide baubles, f', is exactly the same as the frequency that my magical umbrella thingy lobs them out, f0.
f' = f0.
So, now I start steaming towards you at ten miles a minute. I am still chucking out balls at one a minute from my perspective, but the second one only has ninety miles to travel. The third one only has eighty miles to travel. So they hit you slightly quicker each time, right?
If I was running away from you (or more likely, you were running from me) so we instead got further apart from each other by ten miles a minute, then the second ball would have to cover 110 miles, the third 120 miles, etc etc and although I let 'em rip exactly a minute apart, they would take more than a minute between balls to reach you, because each one has to travel progressively further.
Boring maths bit
V = velocity of a christmas tree ball (100 miles an hour) and
Vs = equals the velocity of the source of the balls (ie me), and
V0 = equals the velocity of the observer (ie you, and in this case thats a zero, unless you have begun running away already,) then
See, V/V = 1, always. So if we both stay still, f' = f0(1)
End of boring maths bit
This works for anything with a frequency, like wave forms. That means light, sound.
It doesn't matter what the frequency is at the source (me and my umbrella thingy), if you and I are getting closer, no matter which one of us is moving (could be both), then whatever is being emitted, will accost your senses at a higher frequency than that. White light appears bluer, fire engine nee-naws sound higher pitched. Higher perceived frequency.
If we are moving apart then the perceived frequency drops; white light looks reddish, fire engine noise becomes a lower drone.
Of course, If I was travelling faster than 100 miles an hour, ie faster than my magic baubles, I would break through the ball barrier, and be behind you before you even got hit.
But that's another story.
The posh scientific version here.