A task is a to-do with a date, for example, you have an appointment with the florist, you can add a new task and set the correct date and time. Just click on on the "New Task" button in your Tasks page.
If you want to get notified about the task, you can check the the boxes under "Email Reminders" to tell Marrily when to send you a reminder at the specified time.
(Note that Marrily sends the reminders based on your timezone setting)