Royal Prerogative powers are particular powers of the Crown which have survived into modern times and which are, in practice, exercisable by Ministers who are, ultimately, accountable to Parliament. Some of the powers are highly important and necessary to the efficient workings of government.
As a result of recent litigation in December 2016 concerning leaving the European Union ("Brexit") it came very much to public notice. The litigation was concerned with whether the Royal Prerogative power to make and unmake treaties enabled Ministers to give notice to the Council of the European Union that the United Kingdom had decided to leave the EU. If the prerogative power did not permit the giving of notice then an Act of Parliament was required so that Ministers had the necessary authority. The Supreme Court held, by 8 to 3, that an Act of Parliament was required to give the authority – R (Miller and Dos Santos) v Secretary of State for Exiting the European Union  UKSC 5. As a response to the case, the European Union (Notification of Withdrawal) Act 2017 was enacted.
A brief note about Treaties: