What is the device that used to stabilize rockets and airplanes when linked with a steering system it can help fly on course?

The device you are referring to is known as an autopilot. An autopilot is a system that controls the movement of an aircraft or other vehicle without human intervention. It can be used to maintain a fixed heading, altitude, or airspeed, or to follow a predetermined flight path. Autopilots are typically integrated with the aircraft's steering system and use sensors to measure the aircraft's position and attitude. This information is then used to calculate the necessary control inputs to keep the aircraft on course.