Blog Page Layout: this is where you choose the layout for your blog page.
Archive Page Layout: this is where you choose the layout for your archive page (such as your categories page)
Single Blog Post Layout: this is where you choose the layout for your individual blog posts. Meaning, when you click through to your post..
Excerpt Automatic: choose if you want to display your full post, or a small except of the post on the blog home page. When using excerpt posts, the "featured image" will be pulled for the post image of the excerpt. If you do not have a featured image, no image will display in the post excerpt.