The locate option allows you to find out where the GPS Tracker is at a given moment.
When you press the button, a message will be sent to the tracket requesting it to answer with its current location.
When the tracker's response is received (tipically in a few seconds) the location of the tracker will be saved for future reference and, if you have the application open when the answer arrives, the screen will display on the map where the device is at that time.
In the Android notification bar, an icon will show that a tracker has sent its position. You may open the phone's notification menu to display the location information.
Is you tap over the notification's text, the application will open showing the point on a map.