Social contract describes a broad class of theories that try to explain the ways in which people form states and/or maintain social order. The notion of the social contract implies that the people give up some rights to a government or other authority in order to receive or maintain social order through the rule of law. It can also be thought of as an agreement by the governed on a set of rules by which they are governed.
Social contact theory is sometimes accepted by minarchist libertarians and usually rejected by anarchist libertarians. The latter reason that the notion of a "social contract" is based on the assumption that being born into a given society (which one cannot help) is the basis of that contract, and such a social contract therefore has no validity because no individual ever entered into such a contract of their own free will. See, for example, Lysander Spooner's No Treason: The Constitution of No Authority for a particularly thorough demolishing of this notion.