The app can use the location service with the built-in GPS in your device. It can also connect to an external GPS (and NMEA data stream) using Bluetooth or WiFi.
Your location is shown as a red circle with cross-hairs on the map. The center of the circle flashes to indicate when the position is updated, usually every second or two if there is good GPS reception. If the circle turns grey, GPS reception has been lost. The estimated accuracy of the GPS position is displayed on the data panel, accessed by sliding down the button at the top.
The GPS Lockmenu command scrolls the map to show the position at the center of the screen, and enables moving-map mode. As you move the position is locked and the map scrolls automatically to keep the position in the same place on the screen. You can drag the map to re-locate the GPS position to one side, to show more of the map ahead. As long as you don't drag the GPS position off the screen, it will remain locked at that screen location. If you move off the edge of a map, another map is automatically loaded.
If your device has an app launch button (such as a dedicated camera button), this can be used to lock the GPS location at the center of the screen. Pressing and holding this button creates a Mark at the current GPS location.
If you are moving, you may also see a Velocity Vector. This is an arrow that shows your direction and speed of travel. The length of the vector is scaled to show where you will be if you maintain the same speed and heading for 5 minutes. You can change the time scale of the velocity vector under the GPS Settings.