GPX is short for GPS eXchange Format and a GPX file is a way of sharing route information. It is universal so can be used on many devices such as a GPS, phone or tablet. Once downloaded, you will be able to follow the route on your GPS to help with navigation.
The GPX file is simply a list of data points that need to be interpreted by an appropriate app - it is not a route map and cannot be opened without a suitable reader. There are many GPX readers available - just search in your app store or online.
You can download GPX routes for many of our guidebooks - just check the book page for details.