Cross-selling is an eCommerce practice where retailers offer a user additional products that complement those that they have already purchased or are about to purchase. Up-selling, in its turn, is the strategy that consists of suggesting a user more expensive or upgraded products compared to those that they have initially chosen.
A user wants to buy a cell phone from your company
Suggesting a screen protector, phone case, or headphones
Offering a phone with a bigger screen, better camera, upgraded features, and, thus, a higher price