It depends what your book's about, of course, but generally, your platform should indicate that you are the go-to expert about your chosen subject and that you already command the attention of a book-buying audience. For example, if you are writing a diet book, you should have some sort of nutrition/medical background and probably an affiliation with a respected clinic (or you should be Oprah's trainer). If you're writing a dog training book, you should be Cesar Millan
.
On a smaller level, perhaps you have a high-traffic blog about a particular market niche you're exploring--but it would have to have significant hits. Maybe, as in the example of HACK by Melissa Plaut, you've "lived the life" and demonstrated that people are already seeking out your writing.
People whose queries are rejected because they have no platform are often those who either have no professional credentials or no means of attracting attention beyond what the publishing house can do. It doesn't mean that the actual writing isn't well-researched, articulate and engaging--just that not enough people will pay attention when another John Doe's book is published.