See the image below. Between this internal flash and the on-camera lens is the sensor for measuring the intensity of flash light exposure. When you turn the camera on, the internal flash is set to the automatic mode. That is, when the available light is insufficient for a correct exposure, the internal flash fires. A GN has a unit either in meter m or in feet ft , and is in general calibrated at ISO 100.
Guide number is a given number.
Thus, if you know the subject to flash distance, you can calculate the aperture to be used for a correct exposure. That is, F3.0 should be used for a correct exposure. This is a problem of Coolpix 990, because F3.0 is almost the largest aperture. If you use the tele end of the on-camera lens, this distance reduces to 7.5 ft. Note that the relation of GN shown above does not hold for close-up work.