Upselling is the process of getting a customer to increase the amount of money they are spending once they have decided to place an order. A typical example would be a burger bar asking the customer if they would like fries with their order. If they say yes, then the total order value will increase.
Of course it doesn’t stop there. You may be asked if you want a drink too.
Upselling can work really well if what you offer after the initial purchase decision is highly relevant to the buyer.
Note that upselling is not the same as backend selling which is the process of selling things to customers who have bought from you earlier. An upsell happens at the time an order is placed.