ND8 Neutral Density filter. This dark filter cuts the amount of light reaching the sensor by three stops. On a DSLR, there are several potential uses for a neutral density filter, but with the GoPro, there are two main ones. The first is that if you happen to be shooting in ultra-bright light, this will bring the levels back to help prevent highlights from blowing out. The second is that it can force the GoPro to use a slower shutter speed. Since you don’t have manual control over the shutter speed, this isn’t as flexible as on a DSLR where you can drag the shutter, but ideally, you could use it to smooth out time-lapse footage.