Under the Countryside & Rights of Way Act 2000 (CROW Act) certain areas of land were opened up for public access under a 'Freedom to Roam'. This allows certain rights such as walking over the land. This Access land is shown on the new generation Ordnance Survey maps as a yellow wash and is shown on the ground by the access land symbol, shown below:
Access Land symbol
Full details about Access Land can be found on the Countryside Access website.