Amazon Coins can be used to buy apps and games, as well as items inside apps and games. Since Amazon often has deals on them, you can usually buy them at a discount, making them a great way to save money on games, apps and in-game content. 100 Coins are worth $1 and the more you buy, the larger the discount. (For instance, one popular use of Amazon Coins is to buy Hearthstone packs and adventures with them.)
As you can see, 5,000 Coins–which is worth $50 towards app, games or in-game content–is usually on sale for $45 (10% off), but is sometimes available for as low as $40 (20% off).
10,000 Coins–worth $100 towards apps, games or in-game content–is usually on sale for $90 (10% off), but is sometimes available for as low as $80 (20% off). The historically lowest price ever, however, was during a December 2014 sale, when 10,000 Coins could be purchased for $70 (30% off). That was the best Amazon Coins deal ever offered, but that kind of sale has only been offered once so far.
To see the current price of Amazon Coins, see this link.