It's entirely possible that I'm misusing the term; please correct me.

In game theory and economic theory, a zero-sum game is a mathematical representation of a situation in which each participant's gain or loss of utility is exactly balanced by the losses or gains of the utility of the other participants. If the total gains of the participants are added up and the total losses are subtracted, they will sum to zero.

The secondary market is basically a zero-sum game. For every person who buys a stock, there must be another person to sell the stock. That means that money flows one way and the stock flows the other way. If the price goes up, then the buyer will profit from selling it, but the seller will lose money from buying it back.

In reality, there are more than two people in the market, but the net effect is the same. Each person who enters the stock market brings some money in (and each person who leaves the stock market takes some money with them), but the pool of money floating around the market is finite. For one person to gain money in the market, another person must lose it. Even if you buy a share and its value increases, the only way you realize any profit from it is if someone else gives you money for the share.

This, of course, is somewhat simplified, and I didn't take into account IPOs or dividends, for example.