You can decide whether to approve each appointment manually or automatically. This setting can be adjusted in the Service Method screen by toggling the switch for Automatically Approve Bookings.

When Automatically Approve Bookings is disabled, the customer will receive a customizable message thanking them for their request and informing them that a response will be sent shortly.

Found in the Notifications & Quotes section

To move the appointment from the Pending phase, you must change its status to Approved within the appointment.