When an event is designated as a “private event”, your colleagues will be able to see that you have an appointment but the corresponding details will not be visible. When an event is made private, you will also have the option to select an appointment mode of “busy” or “available”, or "out of office" helping your support staff better understand your availability and schedule your time accordingly.
For example, you may create a private event to remind you to order flowers for your mother’s birthday. For this particular event, you will still be in the office and available to tackle other matters should they arise. For a doctor’s appointment that will call you out of the office for a set time-frame, you should select “busy” since you will not be available.