What does “stimulated emission of radiation” mean?

Stimulated emission is a complicated phenomenon of physics, first described by Albert Einstein. Atoms must be in an excited state (an electron is in an elevated orbit). Normally, adding a photon with energy equal to the energy between orbits would raise this electron to a higher orbit. Instead, in special circumstances found in laser systems, two photons are released from the atom, and the electron returns to its lower, resting state. These two photons are then able to enter two other atoms with excited electrons, allowing rapid multiplication of photons in a process similar to a chain reaction. This process accounts for the stimulated emission of radiation used in the acronym laser.