In the calendar, you can see reminders that you have set for yourself from other directories, such as Contacts (about the need to call back) or Documents (about the need to sign), etc.
Tasks and Deals are also displayed here if their scheduled completion date falls within the selected month.
You can also manually add a one-time reminder or a reminder that will repeat every day, week, or month right from here.
Such quick reminders will not be linked to any directory item.
You can read about how to attach reminders to directory items in the Reminders section.