In the Calendar you can setup availability time windows. This way, when your clients are looking at the online client Calendar to request/book a session with you, they will be restricted by the availability you have set on your Calendar.
It is also useful for your own informational purposes, so you can map out your weekly schedule with your various activities using descriptive text.
When you click somewhere on your Calendar, you'll see the 'Set Availability' button:
This will take you to a form where you can fill in the appropriate details:
These 'Unavailable' and 'Available' time windows will then be plotted on your Calendar:
If you'd like to change the default colours used for the availability time windows, you can customise these in the Miscellaneous Settings area.
When your clients view the Calendar availability through the Client Area or Mobile Apps, they'll only be able to book/request sessions that are available (assuming that you allow online bookings, as set in your Client Area Settings) . The clients do not see the 'Description' text, this is only visible to you as the trainer.