Ordnance Survey grid is a geographic coordinate system used primarily in UK. It uses letter prefixes and digit suffixes instead of degrees to refer to areas of UK. For more information see this wikipedia article.

To input a latitude/longitude, first enter the N,S or W,E symbol after which enter the degrees minutes seconds, separated by space. For example: [W 47 1 32] means 47° 1' and 32" West longitude.

To transform to decimal degrees go to this page here.