This is a quote from John Rawls, a moral philosopher whose work A Theory of Justice has been highly influential since its publication in the 1970s. It is directly relevant to a famous thought experiment used by Rawls to articulate what kind of society would best establish the principle of "justice as fairness." Rawls asked his readers to imagine a scenario in which a person had no knowledge of their social standing, demographic, economic class, or any other information. Thus, as he says in the quote, a person would not be "advantaged or disadvantaged in the choice of principles by the outcome of natural chance or the contingency of social circumstances."
Rawls argued that in this situation, unaware if they would be wealthy or poor, the rational choice would be for a person to choose a society where a government took action to ensure that everyone was treated fairly and had equal opportunity for success, economic and otherwise. Here, Rawls claimed, was a rational basis for a just society. It was the only kind of society a person would choose if they did not know their socioeconomic status. It was also, as this quote suggests, a way to imagine a "state of nature" in which people were actually equal. This concept was foundational to Rawls's argument, which has been foundational to liberal thinking for decades.