A Blueprint is a default Layout for your Page that you set up.
When creating a Page, you need to select a Layout for it which includes Head, Main and Footer. These are always empty upon creation. However, if you create a Blueprint in the Master App, you can add Cells and Tastics within a Default Layout 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.