This feature is the integration of the PayPal Express Checkout facility into a store. PayPal Express Checkout offers online shoppers a simple three-click checkout process if they're one of the 150 million people registered with PayPal, as they don't need to enter their postage and billing information when making purchases.
To enable functionality of this feature, you're required to set up a Website Payment Standard account with PayPal and are responsible for associated/ongoing costs.
Currently the integration of this feature is being funded by PayPal, however this is subject to status and if the funding is withdrawn prior to completed integration, it will no longer be available free of charge.
For more information about PayPal Express Checkout, click here to visit Paypal's site.