What3words divides the globe into a grid of 57 trillion squares measuring three metres by three metres and gives each one a unique, 3 word name.
The app uses the GPS on your mobile phone to display your approximate position on a map, which you can then zoom in on and pinpoint more precisely. It then gives you the 3 word address for the square you're in.
The reason the emergency services recommend What3words is that it can save them precious time getting to the scene of an accident. There are obvious benefits for veterinary surgeons too, not just when called to remote locations, but also when a client's postcode is ambiguous.
What3words is free of charge and can be downloaded for Android and Apple phones. There is also a What3words website, what3words.com, which provides the same service, but that only works online (whereas the app works offline too).
I've reviewed the app in more detail on YouTube here: