When and why it was written

When

Published in 1762, twenty-seven years before the French Revolution.

Why

Rousseau set out to explain how legitimate authority is possible at all, having concluded that most existing authority was not.

What it means

The opening sentence of the book, and it is a problem statement rather than a slogan. Rousseau immediately says he does not know how the change happened but intends to ask what could make it legitimate.

His answer is not the abolition of chains. It is that obligations you genuinely authored are not chains, which is a much narrower claim than the sentence alone suggests.