This is a mathematical concept that turns out to be very important in business too.
A zero-sum game is a game where when someone wins something, someone else suffers an equivalent loss.
So if there is a pie and two eaters, when one person eats a slice of pie, there is less left for the other. Most sports are zero-sum games. An example in business is market share; if you increase yours, someone else is losing market share.
A non-zero-sum game is a game where it is possible for everyone participating to be better off. Using the pie example above, it is like making the pie bigger. In the business example, a non-zero-sum game is about making the market bigger, making everyone’s share more valuable.
More details are available on my Zettelkasten