What are linked products?
Linking products allows you to upsell or cross-sell items in your inventory.
Upsell products are those which you would recommend as an alternative the the one the customer is currently looking at, perhaps because it is more profitable or better quality, for example.
Cross-sell products are those which you would promote in the cart to buy alongside the item the customer is purchasing.
You can add linked products by navigating to Dashboard, Products, then Edit Products. From this screen you can search for products you wish to link together.