The photoelectric effect is a process whereby light falling on a surface knocks electrons out of the surface.

Einstein's paper explaining this effect was one of the earliest applications of quantum theory and a major step in its establishment. To explain this effect one has to consider that light behaves like a stream of particles called photons.

Each electron is ejected by a single photon or light quantum striking the surface.

In the quantum theory, the frequency, f, of the light determines the energy, E, of the photons in that light beam.

E = hf

where h is Planck's constant (h = 6.626069 x 10^-34 Joule seconds). The energy of the emitted electron is given by the energy of the photon minus the energy needed to release the electron from the surface. It thus depends on the frequency of light falling on the surface, but not on its intensity.

Higher intensity light has more photons, and so will knock out more electrons. However, if the frequency of the light is such that a single photon is not energetic enough to release an electron from the surface, then none will be ejected no matter how intense the light.

This phenomenon could not be understood without the concept of a photon, a quantum amount of light energy for a particular frequency. If light were simple a wave-like phenomenon then increasing the intensity and thereby increasing the total energy falling on the surface would be expected to eventually provide enough energy to release electrons no matter what the frequency. Furthermore, in the classical picture one would expect the energy of the emitted electrons  to depend on the intensity of the light -- but it does not.