Explanation
When monochromatic light hits a metal surface, electrons can be emitted. In the classical picture only the intensity would need to be high enough to free electrons from the metal. Something else is observed: no matter how bright the light, electrons only leave once the frequency ν exceeds a threshold frequency ν0. A higher light intensity at the same frequency only changes how many electrons leave, not their maximum kinetic energy Emax.
In 1905 Einstein explained this with Planck's energy quanta. Light delivers its energy in discrete packets, the photons. Each photon has an energy of E = h·ν, where h is Planck's constant. At least the work function WA is needed to free electrons from the metal. The rest is converted into kinetic energy. The photoelectric effect thus shows the particle nature of light.
Emax = hν − WA
In the applet the retarding voltage U lets you determine the maximum kinetic energy Emax an electron has. At U0 no electron arrives any more, then eU0 = Emax.
Simplified model: work functions are typical values. Every photon above the threshold frees exactly one electron, in reality fewer do. Electron energies are spread evenly between 0 and Emax, contact potentials are neglected. After Neamen, Semiconductor Physics and Devices, section 2.1.1.