Cornerstone content is content that is vital to your website, content without which it simply would not be able to function.
Perhaps the best way to think of it is to ask "If I had to choose 6 pages of my website to show to a complete stranger who had never heard of me, which pages would they be?"
If we consider my link2light website as an example those six pages wouldn't include any of my blog posts. They are not fundamental to my business. I would include:
That's it - 4 pages! There's lots of other content there but it is not fundamental to the website - the website could do what I wanted it to do without them.
The purpose of such analysis is to make sure search engines are just as focused as you and understand just as much as you what your cornerstone content is.
And how do you achieve that? Internal linking.
Once you have identified your cornerstone content these should be the prominent options in your navigation menu and at least one should be behind your Call to Action.
This way search engines crawling your website see these links over and over again and understand they are important to you so therefore they are fundamental to the website.
The mistake made by many webmasters is that they get overwhelmed when they have created too much content and end up with confusing navigation menus and site structures while visitors find it hard to track down the key information they are looking for.
I'm Tim Hill, a Search Engine Optimization and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.