A platform communicates your expertise to others. It includes your Web presence, any public speaking you do, the classes you teach, the media contacts you’ve established, the articles you’ve published, and any other means you currently have for making your name and your future books known to a viable readership.
Basically, your platform is everything you do with your expertise. A platform-strong writer is a writer with influence. Once you establish a platform, it can work for you 24/7, reaching readers even as you sleep. Of course, this kind of reach takes time. If many others already recognize your expertise on a given topic or for a specific audience or both, then you likely have an active platform.
I find it helpful to define a platform as a promise writers make to not only create something to sell (like a book), but also to promote it to the specific readers who will want to purchase it. This takes both time and effort, not to mention considerable focus.
Author of Get Known Before the Book Deal & Writer Mama
(both from Writer's Digest Books)