If you have a $100 "in-store credit" (or just "store credit"), it means the shop owes you $100 (perhaps when you returned an item to them), but instead of giving you the $100 in cash, the shop gives you a $100 credit akin a gift card or voucher. So you can use that credit for other purchases in that shop. In this way, the shop encourages you to buy from them.
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