This document discusses how to create a master page in ASP.NET. A master page stores global page elements that appear on every content page, has a .master extension, and allows for consistent styling across a web application. It provides a template that defines placeholders for content pages to fill. The content and master pages are combined to output the final page. The document demonstrates how to create a master page file, add elements to its design and source views, and replace placeholders with content from a content page file.