The political theory that legitimate government rests on agreement — explicit or implicit — among the people it governs, developed by Hobbes, Locke, and Rousseau into the foundational framework of modern constitutional democracy, and contested ever since on questions of who actually agreed, what was agreed to, and whether the theory can justify political obligation at all.
You never signed a contract with your government. Neither did your parents, or their parents. Yet social contract theory — the idea that political authority rests on the consent of the governed — is the foundational principle of modern democratic legitimacy. Every constitution that derives authority 'from the people,' every revolution justified by governmental breach of trust, every argument that citizens have a right to resist tyranny — all trace back to this idea. Understanding social contract theory is understanding why modern governments claim the right to rule, and why that claim has always been contested.