"The Social Contract" is a political/philosophical model that began in the Age of Enlightenment, that is concerned with the genesis of society and it's governing body, i.e. The State. It's key aim is to delineate the rights of The State over the individual, that the individual has conceded to and inadvertently relinquished by being a part of society. The individual, by taking part in society, must surrender some of their autonomy to the magistrate in exchange for protection from The State.
Natural rights and the social concept theory explained
Within this theory, there is some dispute amongst several philosophers, including John Locke (Second Treatise of Government), Jean-Jacques Rousseau (Du Contrat Social), and Thomas Hobbes (Leviathan), over the prevalence of natural rights over legal rights and vice versa. 'Natural Rights' are considered to be God-given rights, that are not contingent upon any societal beliefs or laws, and therefore Locke argued that these rights were inalienable and widely pervasive. Legal rights are the rights bestowed on to an individual by the legal body.
During the same period (the Age of Enlightenment), natural law became a rising phenomenon. Through the acclamation of said rights, the divine right of Kings had been challenged and natural law became an alternative modality for social contract. Locke believed that at the substratum of society, individuals would be morally bound by natural law to come together and form a governing body through fear of mutual harm.
This is how the concept of natural rights ties in with the Social Contract theory.