The payment link defines an URL to automatically create transactions.
Payment links help to collect payments in cases where no backend is available for processing the payments on the merchant side. There is no need for any kind of server side programming by the merchant.
The payment is created by invoking a URL with specific parameters. The parameters are sent via a regular HTTP GET or POST request. For example, the billing address will be read from the parameters provided in the HTTP request to the payment link URL.