_index.md ➥
_index.md has a special role in Hugo. _index.md allow you to add front matter & content to your list templates (e.g. templates of section, taxonomy, taxonomy terms, and your homepage template.)
Also _index.md means that directory is a Branch Bundle (e.g. list template s frontmatter home page, section, taxonomy terms, taxonomy list)
index.md ➥
index.md is a file that is used as the front matter & content of a page. for example below both are same.
content/blog/how-to-use-hugo.md
or
content/blog/how-to-use-hugo/index.md
index.md tells hugo that directory is a Leaf Bundle (This direcotry is collection of content and attachments for single pages)