A platform is a group of people that want to read a certain book or go see a specific movie or listen to some music or etc....
Let's use a movie as an example. You've just finished making a movie, and you want a lot of people to go watch it. To get all these people to watch it, you have to market the movie. You do this by putting trailers of your movie on the internet and on TV. You can also invite some critics to watch your movie so that they can review it (hopefully good reviews). There are many ways to market your movie so that these people will want to watch it. These people, who all want to watch this specific movie, are your platform.
Because of the marketing you've done to build your platform, these people are all excited about going to watch the movie even before its released. They tell there friends, they twitter and facebook it, they make plans to go see it, etc... The movie has a platform of people that want to see it.
This can also apply to novels as well... if you know how to build a platform for your novel.
PS: If you want help building your own platform, then email me so that you can guest post on my blog, or I can guest post on your blog. In other words, we'll link our blogs together, and our readers can read each other's blogs.