This category is for blog posts which are to be shown on both the Main Page and the community messages. Any user can submit content for the front page by writing a blog post and adding it to this category.
To write a blog post, go to your profile page, click "Blog", create a Blog and add "Site Main News" in the Categories panel before clicking Publish. The post might not show up immediately, if it doesn't then click this link: Purge Main Page from Server Cache.
Obviously, only blog posts should be in this category. There may be images and other files related to blogs but there probably shouldn't be any normal articles.