Mantis is a web-based bug tracking system written in PHP and MySQL. It allows users to create projects, track bugs, and manage issues. The documentation outlines how to install Mantis locally, including extracting files, generating database tables, configuring settings, and creating the first project. It also demonstrates how to add users, assign them to projects, report issues, and resolve issues by changing their status. Mantis provides features like summary reports, project documentation, custom fields, and email notifications to help manage bug tracking.