Rakuten is a “shop-through site” in that you sign up for Rakuten then login to their website where you will see a list of top brands such as Nike, Gap, Adidas, Verizon, AT&T, and many more merchants each offering discounts on purchases made at their website. You must log in to Rakuten first then click through Rakuten to the merchant's site to get the cashback. Make your purchase and receive a cashback reward in your Rakuten account.
Currently, you can get $40 just for signing up using my referral code. There is most likely a merchant selling something you were already planning on buying for under $40. Simply sign up, make your purchase, and get $40 free. Once you sign up, you can also refer friends and get another $40.