sitemap_index.xml is the index of your sitemap. That means, it is like a common file linking out to your actual sitemaps. This is the only one you need to submit to Google instead of individually having to submit a new sitemap every time you install a new custom post type or a new plugin that adds its own products/posts.
page-sitemap.xml is a sitemap file containing links to all your pages.
post-sitemap.xml is a sitemap file containing links to all your posts.
Hope that helps and please do not hesitate to let us know if you need our assistance with anything else.