A binary star is two stars which orbit around each other. For each star, the other is its companion star. Many stars are part of a system with two or more stars. The brighter star is called the primary star, and the other is the secondary star.
Binary stars are important in astrophysics because looking at their orbits allows scientists to find out their masses. From this, scientists found the the mass–luminosity relationship, and used it to find the masses of single stars.