The backlight is actually a form of illumination that is present at the back of the LCD that we see. LCD doesn’t produces light itself and requires the support of ambient light or a light source in order to produce visible images that we see on iPhone screen. The back light typically illuminates the LCD from back or sides. iPhone 6s Plus has a LCD screen which is supported by several layered light source. There are multiple technical reasons behind the non functional back light or a dim back light and eventually it needs a repair or replacement through professional help at MyCelCare.