Credits can be used to reduce the price of participating products as an additional discount. (Individual merchants choose whether to participate in the Rewards Credit program, on a store-by-store basis)
Credits can be applied to lower the price of a product, but they cannot be applied to shipping, tax, or any other charges.
There is nothing you need to do to activate Credits. When you check out, any Credits you have will automatically be deducted from your purchase total on any eligible items.
Participating merchants may limit the maximum deduction available on their products via Credits. This is set as a percentage of the product's price, e.g. they may choose to allow Credits up to 35% of a product's price. This percentage may vary from merchant to merchant. That means if you have a $20 Credit and buy a $100 item, then you get $20 off if the merchant you're buying from honors Credits at a 20% level, $10 off if they honor Credits at a 10% level, etc.
Credits do have expiration dates, which represent Eastern Time, and you can always reference those dates by mousing over the sale tag icon near your account settings at the top right hand corner of your top navigation.