The main thing about the Topic Pages are the stories or the “Activity feed”, where you'll see topic related stories from like-minded people that are interested in the same topic as you are.

You’ll be able to take part in the community yourself, and post to all of its followers.

Everyone that follows the topic you're posting on can see your post.

For example:

If you post on the World Capitals topic page, your post will be visible to 1 million users.