Question: What does the theory of photoelectric effect explain?
About the article "Photoelectric effect", section "Experimental observations of photoelectric emission".

Candidate passages:
The year 2005 is significant primarily because of the changes that have occurred in the philosophy of physics over the past 100 years.
These changes began in 1905 with the publication of four papers by Einstein that: explained Brownian motion, introduced the special theory of relativity, and described how the photoelectric effect could be explained by the quantization of light, which helped launch quantum mechanics, and developed "E = mc"2.
These papers are commonly called his Annus Mirabilis Papers because they later defined 1905 as a "miracle year" for physics.
Most physicists agree that the first three of those papers deserved Nobel Prizes, but only the paper on the photoelectric effect would win one.
What makes these papers remarkable is that, in each case, Einstein boldly took an idea from theoretical physics to its logical consequences and managed to explain experimental results that had baffled scientists for decades. 
The first paper proposed the idea of "energy quanta" and showed how it could be used to explain such phenomena as the photoelectric effect.
The idea of energy quanta was motivated by Max Planck's earlier derivation of the law of black-body radiation by assuming that luminous energy could only be absorbed or emitted in discrete amounts, called "quanta".
Einstein showed that, by assuming that light actually "consisted" of discrete packets, he could explain the mysterious photoelectric effect.
The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and, more generally, the assumption of infinite divisibility of energy in physical systems.
Even after experiments showed that Einstein's equations for the photoelectric effect were accurate, his explanation was not universally accepted.
However, by 1921, when he was awarded the Nobel Prize and his work on photoelectricity was mentioned by name in the award citation, most physicists thought that light quanta were possible.
A complete picture of the photoelectric effect was only obtained after the maturity of quantum mechanics. 
In a 1905 paper, Einstein postulated that light itself consists of localized particles ("quanta").
Einstein's light quanta were nearly universally rejected by all physicists, including Max Planck and Niels Bohr.
This idea only became universally accepted in 1919, with Robert Millikan's detailed experiments on the photoelectric effect, and with the measurement of Compton scattering.
Einstein concluded that each wave of frequency "f" is associated with a collection of photons with energy "hf" each, where "h" is Planck's constant.
He does not say much more, because he is not sure how the particles are related to the wave.
But he does suggest that this idea would explain certain experimental results, notably the photoelectric effect. 
Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal. 
Background radiation doses in the immediate vicinities of particles of high atomic number materials, within the human body, have a small enhancement due to the photoelectric effect.