The word is old (from the 17th-century 'dun,' to press for payment) and the practice has not changed much: a fixed cadence, escalating tone, and a consequence at the end you are prepared to act on. What has changed is that it can be run automatically, from your own email, and paused when the customer disputes.
In Reeve
Reeve runs the dunning sequence for every open invoice in your name, escalating one stage at a time and stopping when QuickBooks records the payment.