#=danger (it could go bad and you'll see the "#"'s as appeared then on batman tv shows instead of hits or violence)
The correct answer is that # is the comment character. When the root prompt is #, then the entire command session looks like it is commented out. This prevents disasters if you accidentally copy it and paste a range of the content of the terminal backlog. Or mail a copy to someone who just blindly does a paste into their terminal.