different light waves have different frequencies. if u have a metal and u shine some light onto it, then that light would have a certain frequency.
the photoelectric effect is basically what occurs when light of sufficient frequency or energy shines onto a metal (cathode) and cause electrons to shoot out from its surface.
now, the metal itself has something called "work function" - this is the energy that is required to release an electron from the surface of the metal cathode. for the photoelectic effect to occur, then this means: energy/frequency of light > the work function (so the electron escapes from the metal).
did that make sense or did i just complicate it too much?