Although the assigned user should update the status of a work order to "Complete" when they have finished the work, an Admin can choose to update any work order to the "Close" status without it ever being in the "Completed" status. When they do, then the following Prompt will appear.
Yes will mark the Work Order as "Complete" by you at the current date and time, then set it to a status of "Closed". Note that for the most accurate data, Work Orders representing work that was actually done should be marked "Complete" at the time they are completed, by the person doing the work.
No will simply change the status of the Work Order to "Closed", skipping the "Complete" step altogether. Doing so will result in this Work Order being omitted from the "Completed" data on the Analytics page. This may be desired if the work was simply never done (perhaps because the work was no longer needed).