In 1888, Heinrich Hertz discovered that when light strikes the surface of certain metals, electrons are ejected. This is called the photoelectric effect. To escape the surface, the electron must do so with the energy from a single photon collision. The electron cannot accumulate energy from several collisions with photons. Its features are that electron emission ONLY occurs when the frequency of the light exceeds a particular threshold value (denoted v0).
When that condition is met, the number of electrons depends on the intensity of the light, but the kinetic energies of the emitted electrons depend on the frequency of the light.
The energy of a single photon is described by the equation E = hv, where h is Planck's constant (6.626e-34), and v is the frequency of light. In the model, a photon strikes a bound electron, which absorbs the energy. If the photon energy, hv, is greater than the energy binding the electron to the surface, a photoelectron is freed. NOTE: A photoelectron is just a word used to define an electron emitted by an interaction between a photon.
As said above, the frequency of the light must exceed a threshold value. The threshold frequency is the lowest frequency of light that can produce this effect. Overall, it is a very interesting effect, and it was very important in the discovery of photons.