Hugo will automatically create pages for each section root that list all of the content in that section. See the documentation on section templates for details on customizing the way these pages are rendered.
As of Hugo v0.18, section pages can also have a content file and front matter. These section content files must be placed in their corresponding section folder and named _index.md in order for Hugo to correctly render the front matter and content.
Content Section vs Content Type
By default, everything created within a section will use the content type that matches the section name. For example, Hugo will assume that posts/post-1.md has a posts content type. If you are using an archetype for your posts section, Hugo will generate front matter according to what it finds in archetypes/posts.md.