Face-down (裏側表示, uragawahyōji) means that the back part of the card is facing up, and the face of the card is facing down. This is the opposite position of the card being face-up.
The Main Decks and Extra Decks are placed on their Zone face-down at the start of every game. Cards are usually placed face-down via the act of setting. If a player looks at a face-down card, it is still considered face-down and will be returned to the field face-down once that player is done looking.