The "Accessories" field gives you the option of choosing relevant products to associate with this product, to suggest them to your customers when they visit the product page (if the theme supports it).
Click on Related Product and a search field appears.
Type in the first letters of a product and select it. The product is then added at the bottom of the field.
You can associate a product with as many other products as you deem necessary. Click on the cross to delete an association. An association goes one way only: the related product will not feature an association to the current product in its setting page.
Accessories addition/removal is not automatically saved. Don't forget to click on the "Save" button.