Everything you would ever want to know about upsold products is in the Marketing > Upsale section.
Upsold products are products that you can offer customers during checkout. The two main ideas of upsold products are:
For example, your customer purchases a Hawaiian shirt (the required product). During checkout, you offer them a matching hat (the upsold product).
The "required product" must be in the customer's basket to qualify for the upsale offer(s). In this example, the Hawaiian Shirt is the required product.