Modern vehicles can have up to 100 personal computers on board which feed the information to a central control unit, which is called the on-board diagnostic system. These different computers monitor everything; from the tilt of your vehicle when it is moving, to the amount of oxygen in the exhaust system. The diagnostic system can inform the driver by illuminating a warning light on the dashboard.
If your light is constant or flashing (indicating a serious problem that requires immediate attention to avoid a problem with the engine or damage to engine-related components), you will need your vehicle to be inspected by a technician. These professionals use modern diagnostic equipment to determine where in the system there are faults, whether in sensors or related parts.