Let's look at how lasers work. You probably know that an atom can make a transition from an energy level to a higher energy level by absorbing a photon. The reverse process is known as spontaneous emission. Here an atom in an energy level decays to a lower energy level and emits a photon. But there is another process which is exploited to make lasers work: Stimulated emission. When an atom is in an excited state it will decay to a lower energy state much faster than in case of spontaneous decay if interacts with photons of the same frequency that are emitted in the decay.
If you have many atoms in some excited state then the photons from one decaying atom can ...
A detailed explanation is given.