The GPS polling interval sets how often (in seconds) the app will obtain your location from various location sources. If your mobile device has a connection to global positioning satellites, the app will use the satellites as a location source, as they are the most accurate. If your mobile device does not have a connection to global positioning satellites, the app will use cell tower triangulation, which isn't very accurate, as a location source.
The default GPS polling interval is 15 seconds, which provides a fair compromise between accuracy and battery usage.
The GPS polling interval can be set anywhere between 5 and 120 seconds. A lower number will provide higher accuracy, but will also drain the battery of your mobile device very quickly. A higher number will provide lower accuracy, but will drain the battery of your mobile device much more slowly.