Contact Binary is a type of Binary Star.
Contact Binaries: Tidal forces will get into play when two stars are similar in separation. Since stars are not solid bodies but rather gases, gravity will strip material from one star to the other. Thus we conclude the binaries are in contact, even though their surfaces do not specifically touch.
Why stars swap content is similar to how a ball circles a hill on and off. The ball has to have enough kinetic energy to overcome the hill’s potential energy. There are lineages of equipotential around two stars. Just imagine two lakes nearby. As the water grows it takes on the form of the land’s contours, the equipotential contours. When the water level goes up too high, so the lakes join.
Similarly, equipotential lines occur along stars, where the pull of gravity from one star dominates that of another. This line where the balance of forces or energies is called the Roche lobe. When the radii of the star exceeds the lobe of Roche, the gases are free to pass from one star to the other. Typically in tube shape or stream form.