English As we have already answered above, if you enter the middle, the player wins both bets, i.e. gains a very tangible «advantage». If it does not hit, the winnings will come from only one of the bets.
A positive middle means that in any case, the player will have a profit, even if the winnings come from only one bet. In practice, there are very few of these middles and they are narrow, so sometimes it makes no sense to pursue their positivity specifically.
A negative middle means that if the middle does not hit and only one bet wins, the player will receive slightly less. This is done by those who want to increase the middle and thus increase the probability of winning both bets, thereby paying for the possible disadvantages.