It means that you should not be upset if someone you desire for companionship doesn't share the feeling. You drop a line and try and catch another. In Philadelphia, we say that women (or men, if women are speaking) are like streetcars (or buses, or subways, etc) - wait ten minutes and another one comes along.
I've also heard, but much less commonly, that women (or men) are like Kleenexes - there always another one popping up when you are done with the one you have.
