Once you download the app, it will ask you pick 5 stores you go to regularly. Every time you shop there, cash will magically appear in your account on the app.
Rakuten offers cash back up to 40% of your purchase depending on the store (realistically it mostly ranges from 2-5% which is still a great deal for doing nothing).
There are bonuses for scanning any receipt, and the app will automatically scan your receipt for brands that give additional cash back – no work on your part!