This great feature provides you to fill the checkout form by passing a query string in the URL.
First, take a look at this Sample String:
This example URL includes some extra parameters to fill the Billing Email ID, Billing First Name & Billing Last Name of the customer.
You can provide the exact URL to your customer to make the checkout process much faster and provide pre-filled values on the checkout page entered in the URL.
For Billing Fields
For Shipping Fields
This is all about pre-populating the Checkout Form Data using Parameters.