The Ordnance Survey maps come with a "grid" of blue lines - each with letters and numbers associated with them. This is the National Grid referencing system which allows you to pinpoint exact areas within the UK on a map.
Initially the UK is divided into 100 kilometre squares (represented with letters), and in turn each of these squares is further sub-divided into 10 squares - representing 10 kilometres (represented by 2 numbers), and so on.
The numbers refer to the "easting" and "northing" coordinates on the map, and refer the the bottom left hand corner of a grid square. So, for example. TL63 refers to the TL "square", and within that square, to square six along and three up. You always go "along the corridor and up the stairs" when reading the numbers.
Grid references go up to 6 numbers are are written in eastings and then northings. For example:
TL6537 would be read as TL-65-37 - i.e. Square TL, and then 6 squares along, three up, and then within that square, five along and seven up.
