Discount or coupon codes work in one of three ways:
On the discount codes page there will be a puzzle to solve. The solution of this will be a discount code that will allow a 21% discount off any purchase.
Some individual products will have their own discount code puzzles that you can solve to get a 21% discount on that product only.
Other individual products will have the discount code as part of the design on the product. Once solved, this will give you a 21% discount code on that product only.
Couple of thing to remember:
Only one discount code will work per product. You cannot stack them.
Discount codes do not work on sale items.