How to use the "Confirm" action? What is the confirmation for and what happens when I confirm a reservation?

You will find confirmation settings in HBook > Misc > Confirmation settings. There are 2 aspects for the confirmation to take into account:

•  Do you wish to block automatically the dates when a booking, with or without payment,  is made on your website?

Depending on your situation, you might actually wish that accommodations are automatically blocked when a reservation is received or you might wish to leave it open until confirmation of the reservation. If open, no accommodation will be allocated until you click on "Confirm". It means that someone else can as well go through the reservation process. You might wish to avoid that and in that case you will set that no confirmation is required. Or you might actually prefer to be open to requests and to be able to confirm or decline a request. 

You can as well have different confirmation set up depending on "Paid" or "Unpaid" reservations. Let's say you offer a choice of payment option to your client (deposit online or bank transfer), you might wish to block the dates only when you actually have received the deposit via bank transfer. But block automatically when customer has made the deposit payment online.

• What happens when you click on "Confirm"?

In the list of Reservations, you can see in the column "Action" a green tick that is a "Confirm" action. Firstly, if you have set that a reservation needs to be confirmed before dates are blocked out, then it will block the dates and allocate the booking to a specific accommodation.Secondly, it will send any email template associated to the "Confirmation of reservation", as set in HBook > Emails. So you might be interested in using this action to trigger the sending of a specific email to your client.