Promarket Bias

Finally, please consider the following excerpt taken from Microeconomics: the economic way of thinking, by Paul Heyne:

"The Economist's alleged promarket bias is probably better seen as a preference for those social institutions and "rules of the game" that make exchanges a positive-sum game, a process which all participants derive benefit."

What is a "positive-sum game?"   What are the “rules of the game?”  How can the market process result in “all participants deriving benefit?”