What's the difference between React.ReactNode and React.ReactElement in props?

type PropsWithChildren<P = {}> = P & {
  children?: React.ReactNode;
};

interface LayoutProps {
  sidebar: React.ReactElement;
}

const Layout = ({ sidebar, children }: PropsWithChildren<LayoutProps>) => (
  <div>
    <aside>{sidebar}</aside>
    <main>{children}</main>
  </div>
);