The photoelectric effect is the emission of electrons by substances, especially metals, when light falls on their surfaces.
If you illuminate a metallic surface with electromagnetic radiation that is above a threshold frequency (which is unique to the type of surface and material), the photons are absorbed and electrons are emitted from the surface . No electrons are emitted for radiation with a frequency below that of the threshold, as the electrons are unable to gain sufficient energy to overcome the electrostatic barrier of the work function of the metal