Order Discounts give the shop owner the power to discount orders if certain conditions are met. One of these conditions could be the way in which payment is made. Lets say you prefer payment by Paypal over say Bank Transfer, then you could offer a (% ) Percentage discount off the order total if the customer chooses the preffered option. Or lets say you want to off buy 3 pay for 2 offer. Even a buy one get one free offer. This is all done under Order Discounts.
It can be a very satisfactory to have order discounts, it gives you more control how orders are processed,