From the Super Mario Wiki
The Back Space is a space in the Mario Party series. In Mario Party 3, it only appears in Duel Boards. If the player lands on this space, a dice block appears and the number the player hits will be the amount of spaces the player will have to go backwards. Landing on one in the board Backtrack makes players go in the opposite direction and change the positions of their partners. The Back Space also reappears in Mario Party 9 in which it serves the same function.
On the results screen in Mario Party 3, the Back Space is referred to as the "Reverse Space".
Name in other languages
||Case retour (MP3)
Case demi-tour (MP9)