What does the D on flashcards on Dance Central mean

The D means that move is a "Double-up" Move.
According to the manual, these moves will "double up" the last part of the move.
On a normal move, you would say, reach to the right, then left, then right, then left again. (in four beats)
In a double up move, you would do something twice, on one side.
So you would reach right, left, right, right, (in four beats).
There aren't that many moves like this, and the D is there kinda as a reminder.
You could also remember the D by linking it to the Dougie.