The photoelectric effect is so important since it showed that energy was transfered in discrete packets of energy [Proving that light has a particle-like behavior] and that energy can be transfered by ligth. It allowed the development of modern computation and electronics as well as allowing the creating and use of the photoelectric effect to draw power from solar radiation and more.
The evidence is that under experimentation [thing that cannot be replicated by simply using words] the voltimeter [Not any ordinary voltimeter, a very sensitive one] connected to the metal surface was able to detect a sligth increment of voltage when the metal surface was hit by light. Meaning that when light impacted the atoms that composed the structure of the metal surface some electrons were detatched from the atoms and generated a flow of current, inducing a electric charge.