A Blueprint is a default template for your page that you can create
When creating a page, you usually select the default layout which has an empty HEAD, MAIN, and FOOTER. But you can create a Blueprint in Master that has these sections filled in with cells and Tastics so that you can save time when creating your Pages.
"I created a Blueprint which has my Header, Footer, and the same cell layout I use across all pages so I don't have to add the same things over and over when I create a new page."
Find more info, see the Creating a Blueprint article.