What Rousseau is saying in this quote is that people deserve to be free but that they are chained by the societies in which they live. What he is trying to do in this book is to discuss when it can be legitimate for a society to put its people in "chains."
For Rousseau, a society may legitimately coerce its people only when they agree to be ruled by that society. He believes that people joined together in societies in order preserve their lives and make cooperation between people possible. They willingly made a "social contract" in which they agreed to be ruled by the state in order that their lives can be improved through increased safety and cooperation.
When people are in society, they are "in chains." The society places all sorts of rules on them that limit their freedom. This is what the quote means--it is saying that people in societies have these rules that govern them and limit their freedom. This is something that does not just happen naturally.
So, the quote means that society takes away people's freedom, but please be sure to note that Rousseau is saying that this is a good thing so long as the people have consented to have that freedom taken.