Laser

An energy level diagram show how atoms can be excited (moved to a higher level energy) or not excited (ground state level energy) . Absorbing or emitting a photon is a common way for an atom to gain or release energy. This emission can be "spontaneous" or "stimulated".

To make a Laser (Light Amplification by Stimulated Emission of Radiation) you have to "pump" energy up from an external source in order to excite a large number of the atoms in the active media, and so promote stimulated emission. You also need a resonant optical cavity in order to force the photons to increase the rate of stimulated emissions. A slightly transparent mirror lets a part of this monochromatic light escape.