The fundamental question that Rousseau tries to address in The Social Contract is that of reconciling the freedom of the individual with the authority of the state. On first examination, these two appear to be opposites, and Rousseau tries to understand how both ideas can be accepted and put into practice.
His solution is that of a social contract. Under a social contract, people willingly decide to give up some of their freedom in exchange for protection from the government. However, this means that if the government fails in protecting the rights and liberties of citizens, these are justified in changing it or removing it. This idea is at the core of modern democratic systems.