a way of getting money at a shop when you use a debit card to pay for the things you are buying, in which the shop gives you money which it takes from your bank account:
I got £40 cashback.
a way of reducing the price of a car, piece of furniture etc where the seller says what the price is and offers to give a certain amount of money back to the person who buys it:
Price on the road - £8750. But on top of that, we'll give you £500 cashback.