Biker greeting

I may have a stupid question.. Which is why only stupid answers and no stupid questions are given:-P I hear the biker’s greetings again and again in all Freds here.. that you only greet certain other bikers.. What is this related to? How do I decide who I greet and who I don’t? I don’t seem to have that yet.. I’m ignorant =( But I’m tickling a little bit now.. Yes, I can check wiki too.. but I think I’d rather hear your interpretation =) Greetings from Saarbrü notch