It simply means that someone is attempting a Back Orifice (BO) attack on your system. The person initiating the attack is doing so by sweeping the internet for any computers that have the virus onboard. Since you do not have the trojan resident on your machine, you have nothing to worry about. Also, since your firewall is set up to block any BO attacks, his sweep resulted in no response from your machine. As far as he is concerned, you don't exist.
To more thoroughly answer your question, tofind any incidence of BO infection, you need to use a program like BODETECT v3.5 (available from many download sites). There is a manual method of detecting BO but that only works if the person who sent you the trojan in the first place, didn't modify the original file at all. The automatic method is far easier. Hope that helps!