The ucenter test program is a really simple program that echos the GPS data on the USB port. The data received on the USB port is sent on to the GPS. This way you can send control data to the GPS and you can process the GPS data in the u-blox u-center (Windows) GPS program. Alternatively you may use the RFzero Manager (Windows).

Instead of the u-blox u-center program there are many alternatives also for other operating systems, e.g.

The ucenter RFzero program has nothing to do, as such, with the optional GPS connection on the bottom side of the RFzero board, but data may also accessed on this port.