Building a platform

So you have this book bursting within you. You really, really want to get published. You start lingering at the bookstore tables at major conferences in order to talk to people in publishing about how to do it, and they say, “It’s all about the platform.”
You scratch your head and think, I was not talking about shoes, or boats, or concerts, I was talking about books. What is a platform and what does it have to do with this amazing idea in my head?