"Cancel" button during document upload results in two pop-up choices:

OK = Okay, stop the document upload and close this window.
Cancel = Cancel my cancellation request; do not stop the document upload and do not close the window.

If one is confused about double negatives and mistakenly selects the "Cancel" pop-up choice, the pop-up will disappear and you can get a do over on the original button.

These kind of options are fairly standard across the Internet.