“Pre order” is an option which allows you to sell a product that is out of stock.
Your product should have two options to show as “Pre-order”:
1) Go to: Shopify Admin panel -> Products -> Edit the required product
2) Configure the inventory. Enable the “Continue selling when out of stock” option
3) Set quantity to 0
If you need to show the “Pre-order” label, enable the option in Theme settings ->
-> Product -> Collection page -> Labels for collection pages;
-> Product page and Quick view -> Labels for product pages.