Commonwealth leaders decided in April 2018 that Prince Charles will succeed the Queen as head of the Commonwealth when he becomes king.
The Commonwealth, currently of 53 nations, was started in 1949 by King George VI, the Queen's father. It is not a hereditary position that automatically passes to the British monarch. Commonwealth leaders can choose anyone to be its head.
The Queen indicated that she wanted Charles to succeed her, and the countries' leaders have agreed. [18.04]