Down is a command that moves your character to the next room in the 'down' direction if a valid exit exists in that direction.
In Game Description
MOVEMENT NORTH SOUTH EAST WEST UP DOWN To move about within the game, you generally specify the direction you wish to head. It is possible to move in the four cardinal directions (north/south/east/west), as well as up and down, depending upon your particular surroundings. See also: Enter, Leave