Prep Time:

Located within your Services Management Page, this feature allows you to enter the time it takes the assigned Staff Member to prepare (setup, cleaning/organizing, etc.) before the start of the service. 

Basically, the Prep Time you set in your service is like being added in its duration time. In that case, the booking widget always displays the next available time slot with already allocated Prep Time before it is being displayed for the clients to select.

Buffer Time:

Within the Booking Settings, users can set up a time-frame that will disallow online client bookings. This will change the time between the first available booking time and the current time. 

For example, if the time is 3 p.m. and the booking time buffer is 1 hour, the first appointment will be available starting at 4 p.m. Users can set buffer times from 15 minutes to 7 days from the current time.