SEO grep applies a series of rules to your site in order to analyse it's ease of access to search engine crawlers. By following these rules you will optimally expose your content to a greater audience.
Some metrics, such a domain age and server location, apply to a whole site rather than individual pages. These rules are applied once to your whole site.
Page rules are applied to each individual page on a website. These rules analyse page content and structure, to enable you to determine which areas of your site need focusing on and improving.