A paylink is an online invoice you can email to your customer which contains a link to a payment page. On this page, your customer can add their card payment details (i.e. card number, expiry date, CVC number) without revealing any card details to you, essentially making the experience similar to that of a secure eCommerce transaction when you purchase something online. The customer will get immediate feedback on whether or not the transaction has been successful.
This is a great complement to the card scanner or virtual terminal if your customer is wary of revealing any sensitive information such as their card details or if you are organising your payments via a central administrator for which sending out payment requests from the office via paylinks is simpler. This is also useful for requesting deposits or sending out payment requests to multiple customers (e.g. for an event).