It's just a convention. commands starting with $ are meant to be executed by a user. commands starting with # are meant to be executed by root.

# indicates a comment in shell scripts, it's interpreted the same way on a command line. That's why it's doing nothing. In this case, the # isn't part of the command though. It's to indicate that it's a root shell. You'd see $ for a regular not-root shell. It's part of that bit of text with your username, hostname, and current directory (with most default shell configurations, look up the PS1 Bash variable) that comes before the command you're typing.