To Rousseau, the social contract is an agreement between all the people of a society in which they agree to completely submerge their individual wills and obey the collective will of the people.
Rousseau believed that the average person does not always really understand what is in their best interest. Therefore, they will not necessarily govern themselves wisely. This means that they need to allow themselves to be governed by the common will, which is much less likely to make mistakes. Rousseau believes that the social contract consists of putting all of one’s liberties and one’s powers into the collective. One then gets back exactly what everyone else gets back. This is a more communitarian vision in which people allow themselves to be ruled (even if they do not think at the time that this is best) by the will of the society as a whole.