A zero-sum game is when one participant in an endeavor gains what the other person in the endeavor loses. There is only a finite amount of value to go around. Many people are inclined to think of their activities in this manner. This would be to their detriment.
But first, why are people inclined to think in this manner? Perhaps it starts when one is a child. Obvious situations such as “if he has this toy, then I do not” abound, and the lesson is learned. Also, there are the the games that children play, boardgames and others, that teach this lesson. When they get older, there are grievance mongers (feminist leaders, SJWs and others) in their face. For these people to get and retain power, they need to get their message across easily, and with a maximum impact. This normally is done through appeals to the easily understood zero-sum game. And by engaging in this, they are doing society a great disservice.
For the world is not just one big zero-sum game as the grievance mongers imply. There are win-win situations, where the overused word synergy applies. That is, the whole is greater than the sum of the parts. Marriage is supposed to be a sterling example of this. I will leave it as an exercise to the commenters to describe how this should work in marriage. I will end by stating that in the modern day, far too often the participants in a marriage view it as zero-sum game, which in the end means that they both lose (with the man typically losing more).