The Ordnance Survey is continually surveying Great Britain and updating their mapping.
The maps are split into tiles which range in size from 20km by 20km for the 1:250,000 scale OS Road Map to 5km by 5km for the 1:10,000 scale OS Vector Map Local.
Every three months or so, we get a digital delivery of map tiles that have changed since the last update.
These updates vary in size but are typically 10-15% of the total tiles in a map set are re-delivered to us on each update. Over the course of a year, four updates can add up to a lot of changed mapping.
What that signifies is that a change was made on a map tile, however what it does not tell us without much closer inspection is if the change was simply a spelling correction, or a new bridge.
Below is an image showing which tiles were updated in the 1st of December 2017 Ordnance Survey 1:25,000 scale Explorer map update.