Product Pre-Order allows you to sale your items even if products are out of stock on your store.

Please read full shopify article here which is very well written : Allow Product Pre-Order

Step 1 : Authorizing payment for pre-orders

When selling products as pre-orders, you should not capture payment immediately unless the customer is aware that they are purchasing a product that is not currently available. Instead, you can authorize a payment without capturing the funds immediately. Authorizing a payment at the time of checkout allows you to capture the funds at a later date, when the product is available and you are ready to fulfill the order.

1. From your Shopify admin, go to Settings –> Payment providers.Scroll down to find the Payment authorization section.
2. Select Manually capture payment for orders:Click Save
   
   
   

Step 2 : Allow products for pre-order

1. From your Shopify admin, go to Product.
2. Click the name of the product you want to make available for pre-order.
3. Check Continue selling when out of stock:
   
   
   
   
4. Click Save.