You must have the GPS functionality of your phone turned on. If you do not you will receive a warning that you should enable GPS. To do so from the Android homescreen you can select: 'Menu' then 'Settings' then 'Security & Location' then make sure the 'Enable GPS satellites' checkbox is ticked.
After tapping, the current GPS position of your handset will be recorded and the accuracy of the GPS position will be displayed on the screen (in metres). The button becomes dark grey to indicate data has been recorded. However, you can update the GPS by tapping the button again, if required.