Recurring bill generate a bill each month (or other period where applicable) that is presented for confirmation. Confirmation require you to provide the exact amount of the bill, as well as confirming its due date. Pro accounts can also attach documents.
To confirm a bill, click on the check icon next to it when listed on the dashboard, or press the "Confirm" button if on the bill details page, and then fill in the form that appears.
The "Confirm and pay" action will also add a payment for the bill with today's date.