The reminder function is there to allow you to set a reminder for a number of days before a task is due, for example you may need to check availability of a tool, or a 3rd party specialist. It could be anything that could impact the main task.
Reminders can be added to Planned, Extraordinary or Scheduled task
For Scheduled tasks the reminders are set on the "Task sheet template" for example here is a reminder to check availability of a tool, 5 days before.
For Planned and Extraordinary tasks the reminders can be added, when the task is added from the reminder selection
NOTE: You must have completed any reminders before you can complete a task.
You can also receive a reminder email report read more here