What it actually shows is that energy is quantized. If you imagine light being a series of bullets hitting a metal surface, you might imagine that whether an electron leaves the surface is just a matter of it storing up enough energy from the incoming bullets of light until it exceeds the threshold for the PE effect 'the work function from the surface' But in fact, no matter how long you shine a light on the surface, nothing will happen until the frequency of the light is high enough, then all of a sudden the surface will start to eject electrons. But by increasing the flux of light to the surface at this critical frequency, all you succeed in doing is ejecting more electrons from the surface, you do not affect their ejection speeds. However, if you increase the frequency of the light above this critical frequency, then the electrons will be ejected at increasing velocity.
This experiment is consistent with light operating as bullets, with each bullet carrying an amount of energy proportional to its frequency, which is the essence of Planck's light quantization hypothesis in an arena having nothing to do with black body spectra which is where the idea originally was applied. Einstein showed that the PE effect was a natural consequence of Planck's hypothesis lending more credence to it.