The Pre-order feature allows customers to continue purchasing products even when they are out of stock.
When enabled, the theme will automatically display a Pre-order button instead of the normal Add to Cart button for unavailable products.

To enable Pre-order for a product variant, please follow these steps:
Go to your Shopify Admin dashboard.
Navigate to Products.
Open the product you want to configure.
In the Variants section, select the variant.

Under Inventory, enable:
Track quantity
Continue selling when out of stock

Once the setting is enabled:
Customers can still place orders for out-of-stock products.
The theme will automatically display the Pre-order button/status on the storefront.