This is the standard notation used on Ordnance Survey Landranger maps.
Each reference consists of two letters identifying a 100,000 metre square block then three digits defining the Easting and finally the three digits defining the Northing with reference to the South West corner of the block.
NN166712 is the grid reference for the summit of Ben Nevis. Where you are given the map number ( For Ben Nevis = 41) it is acceptable to omit the two initial letters e.g. 166712. (Instructions on how to read the references are given on the OS maps).