Use related products to display complementary products when the shopper is viewing a product in your store. If the customer is looking at a pair of pants, you might display a shirt and sweater that goes well with the pants.
Related products are set up from the Edit Product/Related Products screen for each product:
Click the [+] next to Stores
Click the [+] next to your store’s name
Click on the Products link
Click Edit for the product you wish to add the Related Product to
Click the Related Products link at the top of the page
Select the Check Box for the products you wish to associate
Click Update
Your have now added those products to the bottom of the product page under Related Products