how does it explain?

Question

How does the idea of photons of light explain the photoelectric effect?

Answer 1

If you visualize photons as being tiny particles with high velocity, they would act like little bullets to knock electrons free from a metallic surface.

Answer 2

Remember what needed to be explained. Light of less than a certain frequency, however intense, did not cause ejection of electrons from the target. Light of above that frequency did.

Einstein's interpretation, for which he got the Nobel prize (he never got one for relativity!), is that light is not continuous but consists of particles (photons) whose energy is proportional to their frequency. A certain amount of energy is required to eject and electron, and this threshold energy corresponds to the threshold frequency. Below the threshold, no effect. Above the threshold, the photons have enough energy and you get the effect.

There is a happy sequel. The theory predicts that the higher the frequency used, above the threshold, the greater the kinetic energy of the ejected electrons (ask yourself why). Millikan did not believe Einstein's explanation, so he set up an experiment to see whether this happened, in the expectation that it would not. When it did, he changed his mind.

That is the nice thing about science. Disagreements can be settled by an appeal to evidence, and faced with new evidence, people are expected to change their minds.