Amazon Cash is a new way to load currency onto your Amazon balance by visiting a participating merchant like CVS, Gamestop, Cumberland Farms, Kum & Go, Kwik Trip, or many others. You then hand over cash and have the store scan a barcode on your phone, which automatically adds credit to your Amazon account. It's a way to use hard currency to buy something on Amazon, as opposed to needing a credit card to shop online.
So in effect, you walk into a store with $25 and leave with $30 in Amazon credit. You have until 12/16/17 to load the Amazon Cash to your account and you have until 12/31/17 to redeem your credit.