Whenever you complete a trade while logged in to the ShapeShift Platform, you will earn FOX back. The amount will accumulate in your account and can be claimed once you reach 100 (or more) FOX.
FOX earned rates are dynamic and change periodically.
In order to claim your FOX, you can follow the instructions here. To move FOX to other wallets or to trade FOX, you will need ETH to pay the gas fees.
Learn more about FOX here.