If a service on the appointment book appears lighter in a section, then that fragment of time is marked as "Available" in either Setup > Setup Services > Setup Service or in the Setup > Setup Worker > Worker Details > Services > Custom Pop Up.
If the Available checkbox is checked in either Setup Services or Setup Worker Service - Custom Pop Up, that indicates that the segment of time is available for other work. This time slot would be offered as a time available to schedule another appointment when booking using Find Appointment, Standing Appointment or when a client books online.
The benefit of the Available checkbox is most commonly used for a color service, where you take 30 minutes to apply the color, 30 minutes to process (which is checked as available) and 30 minutes to wash & blow dry. In the middle 30 minutes, during processing the worker could take a 30 minute haircut, a bang trim or eyebrow wax. There are many ways that this could be used to your benefit, however this is the most commonly used example.