UPRN stands for Unique Property Reference Number. Every unit of land and property in the UK is allocated a UPRN by local government and they belong to Ordnance Survey, which contains the most comprehensive address library in the UK.
Each local council will submit their address register (including the UPRN) to GeoPlace who will collate the data into a national dataset referred to as AddressBase.
GeoPlace and Ordnance Survey have published a paper 'The UPRN - your golden thread' which provides information about the UPRN, what is it, how it links data together, as well as providing some user benefits. Please see attached PDF for more information.