This button allows you to re-center the map on your current location, orient the map in the direction your facing, and displays a compass on the screen. It is located in the bottom-right corner of the main map just above the Tracker.
Tap once: re-center the map on your current location.
Tap twice: orient the map in the direction you're currently facing and display the compass in the upper right corner.
Tap three times: change map orientation back to North-up and remove the compass.
A blue dot on your screen will indicate your present location. If your location is showing, but inaccurate, turn the Location Services off and back on under your device's General Settings. This should re-calibrate the location sensor on your device to give you a better reading.