An upsell is a recommendation for something complementary or an upgrade to the original order.
For example, a restaurant might ask if you'd like to have fries with your burger, and the answer is usually yes.
An e-commerce store can have a similar upsell offer, which is often displayed after the original order was placed, based on what other people who bought that item also purchased.
Using upsells on your online store is a great way to increase your profit margins, revenue, and improve your customer's experience. It will also increase the likelihood that a customer revisits your store and makes additional purchases.