Photoelectric effect: An effect described by A. Einstein that demonstrates that light seems to be made up of particles, or photons. The light can excite electrons (termed as photoelectrons in this context) to be ejected from the metal. Light with a frequency below with a certain threshold, at any intensity, will not origin any photoelectrons to be emitted from metal. Above the frequency, photoelectrons are emitted in proportion to the intensity of the incident light.
The reason is that a photon comprises energy in proportion to its wavelength, and constant of proportionality is the Planck’s constant. Below a certain frequency -- and therefore below a certain energy -- the incident photons do not contain enough energy to knock the photoelectrons out of the metal. Over that threshold energy, termed as the work function, the photons will bang the photoelectrons out of the metal, in proportion to the number of photons (that is the intensity of the light). In higher energies and frequencies, the photoelectrons ejected get a kinetic energy equivalent to the difference among the photon's energy and the work function.