Reaction wheels are devices used in spacecraft to control their orientation in space. They consist of spinning wheels that create angular momentum. When the wheels spin in one direction, the spacecraft rotates in the opposite direction, allowing for precise adjustments to its position. These wheels are essential for maintaining stability and pointing instruments accurately, especially for missions involving satellites and space telescopes. By varying the speed of the wheels, operators can achieve the desired orientation without using fuel, making them an efficient solution for long-duration space missions.