olddocks on "$content_width ??"

What exactly does this do? it seems all themes require to set this variable like this

if ( ! isset( $content_width ) )
	$content_width = 600;

I am creating a theme, where in the options panel, people can disable sidebar. If the sidebar is off, the theme shows posts and pages full width (1000px) else 700px with sidebar.

How do i set $content_width accordingly to this?