An "Add On Item" is an item that can only be purchased with orders containing at least one non add on item. If you're interested in an item, the product listing will show if that item requires other Add On Items to be in your shopping cart before you may purchase it.
Unfortunately, you won't be able to place an order with just Add On Items only. If you try to check out without the required non add on item(s) as well, you will receive an error message. You will need to add more non add on items to your cart to be able to purchase the Add On Items and continue to check out and pay.
If you no longer want to purchase any of the Add On Items in your cart, you can always remove the items from your cart. To learn how, please visit here.