Postcode Anywhere is a paid service providing address completion and validation. Postcode Anywhere supports international addresses, but offers more sophisticated functionality for UK and US addresses.
To enable Postcode Anywhere on your WebStore, you need to create an account with them. Once you have your account, you will have a username, and will be able to create API keys.
You can learn more about Postcode Anywhere and its services here: http://www.postcodeanywhere.com/address-validation-and-verification/
And you can sign up here: https://www.postcodeanywhere.co.uk/register/
In the ‘Enable Postcode Anywhere’ option in the WebStore Manager, you will need to enter your Postcode Anywhere username, and the API key you want to use, separated by a slash (/):
With the option enabled, the ‘Address 1’ field of both the shipping or billing address forms on the checkout page will show a list of suggested addresses that the customer can select. When the customer selects one of these suggestions, the rest of the address form will be auto-populated with appropriate values.