Basically "You're easily replaceable". A new love will make you forget the old one...who most likely left you heartbroken. A variation of this you'll often hear is "Un clavo saca otro clavo"
..and you can use that on girls too. For instance the girl is talking about how she had a bad relationship you can say "No te preocupes Mami que una espina saca otra..." Coming from a gringo they will probably be somewhat surprised in a good way...of course if the EX happens to be around and hear you say this you may end up having to get some real espinas out of your azzzz...