Mercury Go lets you earn and spend Mercury Dollars (see above for more details). You can also pay your Mercury bill through the app.
If you're a Mercury customer you just need to search Mercury Go in your app store, the Apple app store if you use an iPhone or the Google play app store if your phone is Android. If you're not already a Mercury customer you can join here and then use the app once you've become a customer.
You earn Mercury Dollars by accepting and completing challenges. New challenges will be loaded into the Earn section of the app and will tell you how much you'll earn for completing each one.
Once you've earned Mercury Dollars you can spend them in the Spend section of the app by redeeming for Airpoints Dollars™ or as credit on your Mercury bill. You can choose how many Mercury Dollars you'd like to spend by dragging the spend lever down to adjust your balance.
Your Google Fit data is tied to your Google account (not your phone) so giving the app permission to your Google account means it can record your step count to the right place.