It does not stand for anything.
When planning the mission they did not know which day the actual invasion would be on so they called the actual day of the invasion "D" (D Day).
Things that were to happen BEFORE D Day were known as things like "D minus 10" (10 days before the landing).
Thing that were to happen AFTER D Day were known as things like "D plus 5".
So the battle plan would say that troops would have to be moved to Portsmouth on "D minus 2" or that a certain town would be captured on "D plus 4", things like that.
More here
http://www.ddaymuseum.co.uk/faq.htm