A more thorough explanation:

The Royal Marriages Act is a law that was passed in 1772. It states that members of the royal family cannot get married without the permission of the king or queen. This law was created because King George III was worried that some members of the royal family might marry someone who was not suitable, which could damage the reputation of the royal family.

Under the Royal Marriages Act, if a member of the royal family wants to get married, they must get permission from the king or queen. If they are over 25 years old, they must also give 12 months' notice to the Privy Council. If the proposed marriage is not approved by both the king or queen and the Privy Council, it will not be considered valid.

For example, when Prince William, who is second in line to the throne, wanted to marry Kate Middleton, he had to get permission from his grandmother, Queen Elizabeth II. The queen gave her permission, and the couple was able to get married.

Another example is when Princess Margaret, the younger sister of Queen Elizabeth II, wanted to marry a divorced man named Peter Townsend. Because the Royal Marriages Act did not allow members of the royal family to marry divorced people at that time, Princess Margaret had to choose between giving up her royal title and ending her relationship with Peter Townsend. She ultimately decided not to marry him.