Cashback rewards are cashback for a specific shop that you can earn in the app. You get them by inviting friends, by spinning the wheel in "Spin and Win", or sometimes from a friend who gives cashback away to you.
To get your cashback, connect your main bank account to the app. Then shop at the shop shown on your cashback and pay with that connected account. The app recognises the purchase automatically. If all conditions are met, the cashback is credited to your Cashback Sock.