Can you pleas explain to me the social contract theory?

The social contract theory is a theory that talked about the view of a person's moral and/or political obligations are dependent upon a contract or agreement among them to form the society in which they live in. 

it also means, the voluntary agreement among individuals by which, according to any of various theories, as of Hobbes, Locke, or Rousseau, organized society is brought into being and invested with the right to secure mutual protection and welfare or to regulate the relations among its members.

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