English As we have already answered above, if it lands in the middle, the player wins on both bets, that is, they are in the "positive". If it does not land in the middle, one of the bets will win.
A positive middle means that, in any case, the player will receive a profit, even if only one bet wins. In practice, such middles are rare and narrow, so there is no real need to want to find them at all costs.
A negative middle means that if it fails to land and therefore only one bet wins, the player will lose some money. It is favored by those who want to widen the middle and thereby increase the probability of winning on both bets, thus offsetting any potential losses.