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Under some circumstances, an atom in an excited state can be stimulated to drop to a lower energy state when hit by a photon (particle of light). When the atom drops to the lower energy state, a photon identical to the incident photon is released. If nearby atoms are also in an excited state, a chain reaction will be set off, with released photons going on to stimulate the release of even more photons. All of the photons will be identical, meaning they will have the same wavelength, phase, polarization, and direction of travel. If the chain reaction can be maintained a beam of laser light is obtained. Restart.
It is crucial to the operation of a laser that emitted photons are retained to stimulate more emissions. The purpose of the laser cavity (or resonant cavity) is to confine the emitted photons. The laser cavity consists of two mirrors. One of the mirrors is highly reflective and the other is partly reflective. The one that is partly reflective will allow some of the produced laser light to pass, which is the source of the laser beam, and will reflect the rest to maintain the chain reaction.
A model of a laser cavity is demonstrated in the animation. Light is reflected off of two mirrors. If the circumstances are right, the cavity has stability. That is, light will reflect off the mirrors in such a way that it is confined within the cavity. When the animation is initially loaded, the cavity is stable.