The "D" in "D-Day" doesn't actually stand for anything. The "D" is derived from the word "Day" and was used to communicate the day on which a particular military operation began.
The term "D-Day" was initially used because the exact time and date of a military operation was not always known. Therefore, the military would refer to the unknown date as "D-Day", the day before it as "D-1", and the day after it as "D+1".
This most famously happened in the Normandy Landings, when "D-Day" was originally planned for June 5, 1944, but was delayed by 24 hours due to bad weather. Although the military continued using the term '"D-Day" for many subsequent operations, it is now only used in relation to the Normandy Landings.