I checked the book for White People Throwing Gang Signs. It says that it means, "Hey, you've got a cool bike and I've got a cool bike and we're just out cruisin' and livin' life."
Really, I don't know. If my son asked, I guess I'd tell him, "It's a wave that has evolved especially for riders, regardless of who you are or what you ride. It's a unique wave for unique people."