The primary page at a webpage is called a?

Websites are a network of pages, so there is no 'primary' page. On most sites visitors can start from any page.

However, by convention the home page (usually at just the domain name, or /index.html) is the most important page, and has links out to all the other pages, or at least links to each main section.

Having a lot of pages can be very useful for both visitors and SEO - see the Blog Post in links on the importance of having landing pages and content.