Thanks, it's a circular filter added to a lens where as you turn the filter it reduces the amount of light hitting the sensor meaning that I can use longer exposures in daylight. This particular filter is variable from 3 stops of light to 7 stops. So if I have a regular exposure reading of 1/200th second exposure time, dialling in 3 stops of ND filter will make the exposure change to 1 /25th second. Hope that helps explain it :-)