A stock status is a pre-order message that will be displayed when a product or one of the trackable product's option value runs out of stock. You can assign a different stock status (message) to individual products.
List Stock Statuses
Create or edit pre-order stock messages relating to a product's stock level. Default status is Pre-Order.
If Track Stock is enabled in the product this status will be shown in case product quantity becomes zero and Stock Checkout enabled allowing your customers to purchase out of stock products.
As you can see Pre-order message (or pre-order status) is for customer information only and shown only for trackable items with zero quantity in case administrator allow product to Stock Checkout.