Appointments, contacts, and tasks can sync both ways by default between Dynamics 365 and Exchange*. For example an appointment/meeting can be organized in D365 and will be visible in Exchange. Once established the sync is always bidirectional meaning any change on one side will be reflected on the other, for example a change to an appointment in outlook will reflect in D365.
*Note that when referring to Exchange this is Microsoft’s email server solution that delivers email, calendar, contacts and tasks across devices. Outlook is the end user email client. Note that email synchronization is separate to contacts, appointments and tasks.