Well the thing is it's not a gaming specific term; there's plenty of definitions outside Ron's Big Model specific one.
Social Contract Definition
The term "Social Contract" (or "social compact") got its start from Rousseau and those types who defined it as "An agreement among the members of an organized society or between the governed and the government defining and limiting the rights and duties of each." (Wikipedia)
It has since been usefully expanded into meaning "An implicit agreement among the members of a society to cooperate for social benefits," which applies in circumstances where there's not really a government/goverened split (though a strong GM role might qualify...) A RPG group is a small society of its own, and has rules - explicit, implicit, assumed, badly assumed - about how people should act. That is its "social contract."
Example components of the social contract are: Do people drink at the gaming table? Is giving other players tactical advice OK? Are PCs killed without a second thought? Is GM fudging cool or not? All these are things not usually defined in the games themselves, they are part of the 'social contract' of the people playing the game. It can be expectations about behavior in play (no PvP!) and out of play (You miss the game, you get no XP!).
The Gaming Social Contract In Practice
Some groups get explicit about "defining the social contract." This can be helpful to have people on the same page so that expectations aren't violated when their PC dies or someone pops open a beer at the table. It can also be wonky and pointless if taken to extremes and can be a poor man's attempt to impose behavior they want on other players.
The term social contract is grandiose; there is a social contract even if there's totally not one defined; in most places one would assume someone wouldn't show up to a game butt naked - that's part of our culture's social contract, not unique to the game table really. Similarly most people don't feel a need to write down "don't cheat on die rolls," it's assumed.
Social contracts in gaming groups generally only need additional definition when there's an issue that splits/troubles the group and you want to get everyone to have common expectations. "We start on time even if some people are late." Some things can and should vary - one GM may be casual-kill and another may be PC-entitlement; of course people determine this by inspection themselves but you can avoid some conflict by being explicit before it comes up.
It does beg the question of who has the authority to set the social contract. The majority of the group? The GM? Whoever is the most charismatic and talks the loudest? When you attempt to create an explicit social contract to prescribe rather than describe the group's metaphor you can run into some problems.