If you haven’t shared your calendar with colleagues they will only see your availability (free/busy information) when they view your calendar.
As you add an appointment make sure you change the Show As: setting
|Show As:||Appointment type|
|Busy||Meetings and other internal appointments|
|Out of Office||Holidays and external appointments|
|Free||Reminders and planned work|
If you have shared your calendar with your colleagues you can mark an appointment as Private when necessary so that your colleagues can’t see the details.
You can organise your appointments by category. This will mark them with a colour and label.
Apply categories to appointments (GCF tutorial)