GPS stands for Global Positioning System. Your phone and the iFootpath App use the same satellite system as a car navigation system. GPX, or GPS eXchange Format is an XML schema designed as a common GPS data format for software applications. It can be used to describe waypoints, tracks, and routes.
The format is open and can be used without the need to pay license fees. The file can contain the location, elevation, and time information and can, in this way, be used to interchange data between GPS devices and software packages - like that used with iFootpath. The GPX file can be viewed (and edited) in its raw format using a text editor - if your text editor will not open the file change the extension to .txt
You can read more about GPX on the Ordnance Survey website -
You may also like to read iFootpath Expert: The Joy of GPS