"God Save the King" is the National Anthem of The United Kingdom and New Zealand, whilst also being the royal anthem of various other Commonwealth Realms across the world, like Australia, Canada and Jamaica.
The origin of the tune is unknown, with it not being attributed to an author. The song was adopted in 1745, with it's lyrics changing a lot throughout the centuries.
The song is also known as "God save the Queen" when the monarch is a female, and all uses of male pronouns in the lyrics are switched to their female counterparts.