Upsell refers to when you encourage a customer to buy a little something extra during the closing of the deal (checkout). For example; a shoe salesperson might suggest that when you buy a pair of shoes that you also use some weather protectant spray. These "Upsells" are usually small purchases that the buyer doesn't have to put a lot of thought into and are presented in a timely fashion; right when they have their wallets (credit card) in hand. It's much different and more effective than simply displaying "related" or "suggested" item(s) at the bottom of the view cart.
The Upsell After Feature appears with your configured "Upsell" item(s) displayed. It will also include your custom message promoting the additional item(s). Since the customer has already entered their payment and shipping information, this additional purchase is a simple "click" of a YES or NO button. Whether or not a customer chooses yes, no, loses their internet connection, or walks away and forgets about their purhase, you will not lose the already finalized order.
Imagine being able to increase your revenue by ~10% simply through Upsell After. This could be the difference between a break even e-commerce business or a highly profitable one. In today's competitive advertising arena for e-commerce related sites you can not afford to leave the revenue on the table that can come from Upsells.