You can discover coupons either inside the app or inside Safari.
You can use the Coupons at Checkout for iOS app to search any store you're planning on shopping at to check for current online and in-store discounts.
For online promotions, simply tap a promotion to reveal the code, then tap the code to start shopping.
For in-store promotions, tap the promotion and you'll be taken to more information about the promotion. You may need to show the cashier in the store.
In Safari you can use the "Find Coupons" button at anytime. If you aren't in your shopping cart or at checkout you can still use it to find coupons. Tapping a coupon will copy it to your clipboard. If you're in your cart or at checkout, it will automatically fill in the promo code box. Simply apply it to your purchase.