Thank you. You can use Rakuken in store too. On their website go to the "in-store cashback" tab and you'll see the eligible stores. I'm only showing 89 in my area but there are hundreds more you can use when shopping online.
As far as Rakuten, it's a typical cashback website where you make purchases and get cashback. The amount depends on what the going percentage is for the store you're shopping from. So, for example, I see Macy's is 6% cashback right now. If I were to buy a product $40 or more, I'd get the $40 cashback + 6% cashback off the $40 purchase. If you were to purchase again of course you wouldn't get another $40 cashback, but you'd get 6% cashback since that's what they are giving for Macy's purchases right now.