These deposits are treated as a form of customer credit in your reports. Until they have been redeemed they are treated as a liability and will show in the Total customer credit issued for the period section of your sales reports.
Once the deposit has been redeemed during checkout, the value transfers into revenue and will be reflected in your sales report totals. This will help ensure that the full value of the appointment is recognised at the point of checkout, rather than at the time the deposit was initially taken during online booking.
For more information liabilities, see Understanding liabilities versus revenue.