it's a notational way in the chess program (written by Ken Thompson) to describe a chess move, "pawn from Queen's 2 to Queen's 4."
A very common opening move that "puts a pawn in the center, controlling the important e5-square, and opens the line for the Bc1."[1]
The notation is old. Modern notation would just write it as "d4" because there's only one piece (a pawn) who can move to that square as the first move and only one spot from which it can move (d2).
See the "chess notation examples" table. The password doesn't match any chess notation, but it's close enough that it's obviously (to me) intended to be a chess move. In particular, it moves the pawn in front of the queen (in the initial position) forwards two spaces.
