Einstein showed that light is composed of discrete bundles of energy called photons. Quantum theory also shows that there are certain distinct energy states that any particle may occupy. Unlike the macroscopic world, quantum mechanics do not allow continuous distributions of energy. Instead, energy levels are characterized by the solutions to Schrodinger's equation. Consider an electron in an atom.
Electrons normally gravitate towards the lowest possible energy state called the ground state.
However, an electron may be elevated to a higher state by the absorption of energy. The promotion of an election to a higher energy level is called absorption and the decay and release of a photon is called emission. A gas laser uses these principles to produce an amplified beam of light. The electrons absorb this energy and are promoted to state E 2. This process is called pumping.