A maser is a microwave amplifier by the stimulated emission of radiation, an amplifier similar to the laser but which operates in the microwave region of the electromagnetic spectrum and serves to receive very weak signals. The word derives from the acronym in English MASER, meaning Microwave Amplification by Stimulated Emission of Radiation.

Its operation is based on the phenomenon of stimulated emission of radiation, enunciated by Albert Einstein in 1916. At the University of Columbia, United States, Charles Hard Townes proposed its existence in 1951 and in 1954 built the first.

When a molecule or atom is in an adequate energy state and passes near an electromagnetic wave, it can induce them to emit energy in the form of another electromagnetic radiation with the same wavelength and polarization, reinforcing the passing wave and triggering a cascade of phenomena that lead to greatly increase the intensity of the original impulse.

In some clouds of interstellar matter excited by the radiation of nearby stars, the same phenomenon occurs, which leads to the formation of an intense beam of radiation with a well-defined wavelength and polarization (cosmic masers).

More Info: en.wikipedia.org