HSI are systems that give the pilots information regarding aircraft's horizontal position with respect to the navigation situation. They can even be manually operated and also provide autopilot outputs. Such horizontal situation indicator can also include aircraft compass.
Aircraft HSI uses magnetic field to get information regarding horizontal situation and this information is then provided to the pilots. The HSI also features internal lighting. Some of them are also equipped with anti-vibration apparatus thanks to which they can be installed in helicopters as well.