The Restoration was an time in the history of the British Isles that started in 1660. In 1660 the House of Stuart was restored to power after the Wars of the Three Kingdoms. The Restoration came after The Protectorate, when Oliver Cromwell and his son Richard Cromwell had led the government. The kingdoms of England, Scotland, and Ireland started to have a monarchy again. "The Restoration" means both the events at the start of Charles II's reign and the period of time after they happened.
Charles II came back to England after his exile and he became king of England, Scotland, and Ireland. Before this, the British Isles's monarchy had stopped after Charles I's execution. The three kingdoms became a republic, without a monarchy: the Commonwealth of England, Scotland, and Ireland.
Literature and culture of the Restoration period can also have the name "Restoration". (For example, Restoration drama is the theatre and plays of the Restoration period.)