What is a regency, and how would it work if King Charles' health worsens?

Prince William and his father King Charles III (John Stillwell/PA)

With Britain's King Charles being treated for cancer, his ill health has raised questions about whether there could be a need for a regency in the future.

If King Charles was incapacitated, his son William could be appointed Prince Regent through the Regency Act 1937, taking over his duties.