It means that someone has either forged your address as the 'From' address because it's spam, or perhaps has simply mistyped an address and got yours by accident - the former is more likely. If it's spam I'm afraid you may get some more of this for a time; spammers often send out a buch of emails with a forged address but they usually move on to another innocent address after a time. Just bin it and any like it and don't worry about them.