The GPS has a built in real time clock, which can keep track of time even when it power is lost and it doesn't have a fix yet. It can also help reduce fix times, if you expect to have a flakey power connection (say you're using solar or similar). To use the RTC, we need to install a battery. We provide the holder but the battery is not included. You can use any 12mm coin cell - these are popular and we also carry them in the Adafruit shop.
Normally, if the GPS loses power, it will revert to the factory default for baud rates, configuration, etc. A backup battery will mean that those defaults will not be lost!
The backup real-time-clock circuitry draws 7 uA (0.007 mA) so a CR1220 will last 40mAh / 0.007mA = 5,714 hours = 240 days continuously. The backup battery is only used when there's no main 3V power to the GPS, so as long as it's only used as backup once in a while, it will last years