Metalsmith is a static site generator that allows developers to create custom sites with ease. It's known for its simplicity, pluggability, and ability to imitate other static site generators. Metalsmith can be used to create a simple site with continuous deployment on Netlify using GitHub. The guide provides step-by-step instructions on setting up a basic Metalsmith demonstration site, including creating a folder structure, installing plugins, configuring the build file, and deploying the site to GitHub and Netlify.