Monolith is a CLI tool that saves complete web pages as a single HTML file, embedding CSS, image, and JavaScript assets. It can be used to replace multiple open tabs with a single HTML file, making it easier to store and share web pages. The tool does not require a web browser to be installed and can be used to save pages offline. Monolith has various options for customizing its behavior, including excluding certain types of assets and setting a custom base URL.