The Closed Date field will show you the date when the order was approved (eSigned) by your customer or converted to and Invoice (Completed), giving you one date field on which to filter on in Views and Reports.
By default the closed statuses are Completed, Closed and eSigned however this list can be edited and changed to suit your business needs.
The setting itself can be found by going to Settings > Configuration > Additional Settings > Sales Order Settings and change the values in the List of closed statuses for Sales Order:
Important: Going from one Closed status to another Closed status will trigger a date stamp in the Closed Date field overriding any previous date that was stored there. So the closed date field will have the date of the last closed status that was selected.