"No man is an island," wrote the English poet John Donne. "Hell is other people," wrote the French philosopher Jean-Paul Sartre. Most of human history was an attempt to reconcile these two seemingly irreconcilable truths.
Technically speaking, a man could survive on his own. He could hunt for food, construct shelter, and if not reproduce, at least produce a reasonable facsimile of a mate using twigs, branches, and two coconuts. But we were social creatures who functioned best living in close proximity to one another, and living in groups vastly improved both our safety and our gossip. So we banded together into communities governed by shared sets of morals and values. Such a group was referred to as a society. It was the central organizing unit of our civilization, and the catch-all scapegoat for any part of your life that didn't work out the way you thought it would.
A society was simultaneously one unified mass and a large collection of individuals. Therein lay the tension; for to be part of a group was to exchange certain individual freedoms for certain other collective benefits. These benefits derived largely from economies of scale. A city of five million people is far better equipped to construct, say, a working mass-transit system than an individual family of four. It just is. But in exchange for the mass-transit system, each member of society was expected to adhere to rules regarding its use, i.e. not masturbating on it. Such harsh, at times maddeningly frustrating trade-offs were the basis of the social contract, a document that differed from other contracts only in that you had no choice but to enter it, and it didn't exist.
The social contract provided a framework in which human interaction could occur on a reasonably fair basis. For example, let's say Person A owned a slave that Person B wanted. Were they not inhabitants of a civil society governed by law, Person B could have simply stolen Person A's slave through brute force. Such an existence would have been cruel and terrifying for all members involved-Person A and Person B. But for members of a society, violence was forbidden. Person B was allowed to purchase the slave, but if he stole him he was guilt of a crime, and had to repay Person A the dollar value of the human being he had stolen. This was how a civilized society operated.
This is not to imply the social contract was fixed or unchangeable. There were thousands of different contracts. Some put power in the hands of one man; others, in the hands of thousands of men. (It was usually men though. That part rarely changed.) Each contract itself was subject to constant change and revision; a society that failed to periodically amend outdated provisions regarding how to dress or how much to subjugate women and minorities could find itself the laughing-stock of the diplomatic community. Unless it happened to be strategically situated atop vast reserves of natural resources.