Short answers, pulled from the story.
The photoelectric effect is the emission of electrons from a material when struck by electromagnetic radiation, such as ultraviolet light. It matters because it proved that light travels in discrete energy packets called photons, contradicting classical wave theory and providing a cornerstone for quantum mechanics.
Albert Einstein explained the photoelectric effect in 1905 in a paper titled "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". He proposed that light consists of discrete energy packets, each carrying energy proportional to the light's frequency. He received the 1921 Nobel Prize in Physics for this discovery.
In 1914, Robert A. Millikan made highly accurate measurements of the Planck constant using the photoelectric effect and confirmed Einstein's prediction that photoelectron energy rises linearly with light frequency. Millikan was awarded the Nobel Prize in 1923 for this work, despite having initially considered a particle theory of light "quite unthinkable".
The threshold frequency is the minimum frequency of light required to eject electrons from a given metal surface. Below this frequency, no electrons are emitted regardless of the intensity or duration of exposure. Above it, the maximum kinetic energy of the ejected electrons rises linearly with frequency.
Gilbert N. Lewis coined the term "photon" in his letter "The Conservation of Photons", published in Nature on the 18th of December 1926.
Applications include photomultiplier tubes used in low-light detection, night vision devices using materials such as gallium arsenide, photoelectron spectroscopy for determining elemental composition and electronic band structure, and early television cameras such as Philo Farnsworth's image dissector. The effect also causes spacecraft to develop positive charge in sunlight and lifts lunar dust off the Moon's surface by electrostatic levitation.