The Checkout tool in the Storefront API includes the information that's required for a customer to complete checkout and pay for their order. 

You can use checkouts to incrementally build and update a draft of the customer's order details before completing the checkout and attempting billing. After a customer's payment is successfully processed, a corresponding order is created in the Shopify admin. 

A checkout is used to manage a user's cart as it transitions to a paid order. This process includes specifying which line items are included in the checkout, attaching a customer's shipping and payment details, and calculating tax and shipping rates.

PREREQUISITE:

1. You're familiar with querying products and collections.
2. You've created products and product variants in your store.
3. You've turned your app into a sales channel and requested and been approved for payment processing. 

STEPS FOR CHECKOUT:  

• Step 1: Query product variants
• Step 2: Create the checkout
• Step 3: Update the checkout
• Step 4: Associate a customer
• Step 5: Complete the checkout