In A Theory of Justice, Rawls argued that a society ought to be based on the ultimate ethical foundation, that of justice. A just society is one in which:
...no arbitrary distinctions are made between persons in the assigning of basic rights and duties and when the rules determine a proper balance between competing claims to the advantages of social life.
Essentially, then, Rawls is interested in fairness as a principle of social justice. But he also is concerned with the limits that a society must place on some people in order to achieve generally-agreed upon condititions of justice in a society. So he attempts to propose a theoretical concept of society based on a rational choice. In order to make his argument, he employs a famous thought experiment. Proposing an "original condition" wherein there is no society, he wonders what kind of society a person in this condition would choose. But to complicate the question, he asks the reader to imagine that the person in the original condition is shrouded in a "veil of ignorance" of what their role or station in the society will be. They do not know if they will be rich, poor, of high birth or education, and so on. In this condition, Rawls argues, the only rational choice is to choose a society where the people on the lower end of society are well-cared for. After all, as far as Rawls's imaginary figure knows, he might wind up in that situation. In this way, Rawls attempts to find a rational basis for an ethical society founded on the principle of social justice:
Since it is not reasonable for him to expect more than an equal share in the division of social primary goods, and since it is not reasonable for him to agree to less, the sensible thing is to acknowledge as a first step a principle of justice requiring an equal redistribution.
Rawls is not arguing that forced redistribution of wealth is desirable, or that absolute equality should be the goal of society. He is pointing out that the ethical precept of fairness and justice ought to guide our thinking about society even as natural inequalities between men unavoidably develop.