Rawls is suggesting that the theoretical people in his thought experiement would have no foreknowledge of what was good as an abstract principle, which seems to be what this question is asking. They would not be steeped in any ideologies, and wouldn't be influenced by, for example, Marx, Locke, Bentham, Smith, or any other thinkers to make their decision. Their decision is purely being made out of self-interest, and the crucial point is that they would not have any foreknowledge of what their position or status would be in that society. As Rawls says, this "original position" is similar to the concept of the social contract, a longstanding foundation of Western thought.
Among the essential features of this situation is that no one knows his place in society, his class position or social status, nor does anyone know his fortune in the distrubution of natural assets and abilities, his strength, intelligence, and the like. I shall even assume that the parties do not know their conceptions of the good or their special psychological propensities.
Under these conditions, Rawls tries to show, people would choose a society which was equipped to provide the most possible economic equality, since they could not be guaranteed that they would be among society's elite. If one is as likely to wind up underprivileged as privileged, then a rational decision-maker will opt for a society that best supports the underprivileged. Thus Rawls provides a concept of social justice that proceeds from self-interest, rather than from communal interest. In the "original position," the members of society are not only "autonomous," but they are "conceived as not taking an interest in each other's interests." A just society, then, could have its foundation not in civic-mindedness or altruism, but in rational self-interest. The good that the society conforms to is not predetermined, or influenced by any external factors, but rather the product of rational calculation when presented with the problem.