what does complete submission do?

That happens when you have integrated your form and your submitter closed the page without completing the payment, all these submissions will be flagged as incomplete. 

When you manually complete a submission, then you will see that one listed in the submissions grid, when you manually complete a submission then the payment will not be processed. 

You may also get more info about incomplete payments on this guide: https://www.jotform.com/help/135-How-to-view-incomplete-payments 

Please, if you have any question, let us know.