The photoelectric effect is a phenomenon where light shining on a material causes it to emit electrons. When photons, which are particles of light, hit the surface of a metal, they can transfer their energy to the electrons in the metal. If the energy of the photons is high enough, it can free the electrons from the material. This effect was crucial in the development of quantum mechanics and was explained by Albert Einstein in 1905. He proposed that light behaves both as a wave and as a particle, leading to a better understanding of the interaction between light and matter.