A brown dwarf is a protostar that never had enough dust and gas accreted (roughly 0.05 solar mass)
to achieve a temperature hot enough to ignite fusion. It has low mass (it’s not very big), and low
luminosity (it doesn’t shine much). It's bigger than a planet, but smaller than a regular star.
Most brown dwarfs usually occur with another main sequence (medium mass) stars, kind of like a
companion. Many stars occur as a binary (two) star system. Brown dwarfs then often occur with
another star. The brown dwarf took option 1, and the other star that achieved fusion took option 2.