Your "Duration" field calculates the difference between Start & Finish dates, while your "Total Duration" field sums up the duration of sub-tasks under a Summary task.

Did you inherit the schedule? Just curious as to why you would add a field to the schedule if you don't know what it does.

BTW, I want to believe your "Total Duration" is a custom field and not a default Microsoft Project field as it is not listed in the list of duration fields. Duration fields - Project