The national anthem of Great Britain is a song that is (right now) called "God Save the Queen." When the current Queen dies, she will be replaced (presumably) by her son, Prince Charles. At that point, the national anthem will be "God Save the King." It is, of course, the same song but with just the one word changed.
This song dates from the 1700s. It is said to have first been performed in 1745. It is not clear when the song was first considered to be the national anthem of the United Kingdom.
The national anthem of the United Kingdom is 'God Save the Queen'. which is sung to the same tune as the national anthem of Liechtenstein. It is also used as the 'Royal Anthem' in countries such as Canada, Australia and New Zealand, which still have Elizabeth II as their head of state.
However, each of the constituent countries of Britain have their own anthems which are used at sporting events etc. although England usually retains God Save the Queen.
The word 'Queen' is changed to 'King' when the monarch is male. The word 'save' is best understood in a modern context as meaning 'maintain'. The monarch never sings the words to the anthem, although all other members of the royal family do.