If the Ticket tab is selected in the left panel this tab displays a list of task connected to the ticket. You can edit tasks, e g add new comment, and you can add new tasks by using features in the list.
Tasks and task list are described in section Tasks.
If you select User or Organization tab in left panel the Task List will list tasks connected to the User or Organization respective. Task is included in the list if the User or a member of the Organization has created the task.