The laser is based on the phenomenon of stimulated emission. If an electron of a given atom is in an excited state and if a photon of proper frequency interacts with this electron, the electron is triggered into de-exciting itself by emitting a photon. Moreover, the original photon continues on its way, unchanged. This process continues in a chain reaction causing a ``photon cascade''. When the cascade is sufficiently intense, it bursts through the partially reflecting mirrors of the laser cavity. This burst perfectly coherent since the photons are all in phase. Since the photons have the same frequency, the light is essentially monochromatic.
Note: Due to the design of the lasing cavity, the laser beam is collimated and therefore there is little loss of energy flux due to divergence of the light beam. The 0.5 mW He-Ne gas laser produces about 1/4th of the energy flux of the sun. Although this energy level seems to be below that which causes permanent eye damage, in order to insure safety, never look directly into the laser beam or its mirror reflection.