When a user completes a form, they are indicating that they have completed their part, and the form is ready for the next step in its workflow. This can either mean that the form has been fully filled out and its status will be changed from Draft to Completed, or that the form will be transferred to the next person responsible as defined in your form workflow.
When a user reopens a form, they do so to edit a form that has previously been completed; this will always change the form status from Completed to Draft. See here for more on form statuses and what they mean.
Completing a form
When a user taps or clicks the Complete button, one of the following things will happen:
Form status changes from Draft to Completed: This is the most common case. See here for more on form statuses.
Form is transferred to another user: If your form has a workflow event set up that auto-transfers the form to another user upon completion, then the form remains in Draft status and gets transferred to that user.
Reopening a form / Editing a completed form
Reopening a form can often be useful, particularly if you've found a mistake in a form that you or someone else previously completed. However, note that depending on your user permissions, you may not be able to reopen/edit completed forms. If you DO have sufficient permissions to do this, you can do it in one of the following ways.
In the mobile app: Open your completed form in the mobile app Form Editor, then tap the Reopen button at the bottom.
In the web interface: Open your completed form in the Web Form Editor, then select Reopen from the Form dropdown at the top.