This plugin does is allow people to create posts and pages composed of other posts and pages. All you have to do is insert markup like this, for example, inside a page: [[<post_ID>]]
This will add the post with <post_ID> to that page. This was already the normal behaviour of the Inline Posts plugin. Now here comes something new.
What if you wanted to build a page composed of all posts in a particular category? Just use this notation: category[<cat_ID>]], where <cat_ID> is the category ID to get the posts from.