The S in laser stands for stimulated.
Stimulated means that the photons are amplified by stimulating an
atom to release more photons. An atom can exist in an excited state,
similar to a bow when it is stretched. When the atom relaxes it emits
This is similar to releasing the string of the bow and letting the arrow fly. Normally, an atom will relax from its excited state anytime it feels like it. However, if another photon comes by that has the same energy as the atom in the excited state, the atom will decide to give off its photon and have it join the other. The atom is stimulated by another photon to release its photon.