It is a "National Grid Reference". It is physically marked on the object the node represents, and is a more accurate expression of the location than its lat-lon derived from the map. 

I can't find full details quickly, but you could have a look at https://wiki.openstreetmap.org/wiki/Converting_to_WGS84#Great_Britain and https://www.ordnancesurvey.co.uk/documents/resources/guide-to-nationalgrid.pdf 