Political parties write a platform to tell the voters what they will do when they are elected. Similarly, an author writes a platform telling a potential publisher what they can do to help sell the book. But unlike political platforms, when you are "elected" you will be expected to come through on your promises.
Once they print the book, you and your publisher have a combined goal--to sell as many books as possible. So if you and Stephen Colbert went to college together and he has promised you an interview on "The Late Show" you have just seriously improved your chance of being published.
You should expand upon why your education or life experiences qualify you to write this book. Explain why you are better qualified than others to expound upon this subject. Answer the question, "What makes you an expert?"
Can you get someone important, respected or nefarious to write your jacket note? We need to know that.
What organizations do you belong to and can you arrange to speak at their meetings or conferences or annual assemblages? Do they have a magazine and will they publish a review?
Where can you set up book signings and how many can you do? Can you travel?
Sure, your publisher will have a marketing department and this is all, ultimately, their job. But you improve your chances of being published if you can make yourself a valuable asset.
Remember when we discussed the logline, Random House told us, "We look for provocative, controversial, different, fun, funky, edgy, different—not all of these. Any of these." At this point we want as many of the rest as we can get.
Your platform is another living document that you and your agent will continue to improve upon as your mutual experiences grow. On the next page we discuss the proposal of which your platform is just a part.