This is the blog page. It lists all of the blog posts on your site. The Legumeinfo Jekyll theme does not provide a blog page since not every site may want such a page, but a blog layout is provided so sites that want a blog page don't necessarily have to implement one from scratch.
The posts listed on this page are Jekyll posts. When the site is built a separate page is generated for each post. A blog post layout is provided by the Legumeinfo Jekyll theme for these generated pages.
The blog posts card on the right side of this page is not actually specific to this page. The Legumeinfo Jekyll theme supports adding this card to any page via YAML front matter.