In the Details field you find Add reminder. You use the feature if you want to add a reminder for a specific Ticket if you, for example, want to be reminded about answering the Ticket.

Click Add reminder and pick a date.

You will see the date on the same place.

When the date that you picked has come, a number is shown next to the little bell in Tickets. Only the agent(s) that is assigned to the Ticket sees the number next to the bell.

Those who aren't assigned to the Ticket but are in the same group/mailbox and can open the Ticket, see the reminder if they click the bell. This is where you see all Tickets with reminders listed.