Brown dwarfs are very dim, glowing objects that never gained enough mass to become fully fledged stars. In other words, the temperatures and pressures at their centres never got high enough for nuclear fusion to start.
Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars, i.e. somewhere between 12 and 75 times the mass of the planet Jupiter. Whilst most have been found orbiting stars, some have been found roaming alone in the cosmos.