This is the intro of your blog post. If your post is short, just stick it all in here. Everything in the "Main" block area will be displayed from the main blog list page.
If you tend to write long blog posts that go on, and on, and on - you should consider keeping that content in this "Blog Post More" block area. This area only shows up to visitors who click on the blog post's title or more link to read the full post.
Keep your long posts in here and just a short synopsis up top if you'd like a nice scannable blog.