The King reigns, but he does not rule. Ruling is done by his government, and as head of state in the UK the King is constitutionally obliged to follow the government’s advice. His main functions as head of state are to appoint the Prime Minister, and all the other ministers; to open new sessions of parliament; and to give royal assent to bills passed by parliament, signifying that they have become law. The King also chairs monthly meetings of the Privy Council, to approve Orders in Council; he receives incoming and outgoing ambassadors; he makes a host of other appointments, such as the senior judges, but in all this he acts on the advice of the government.
