Istanbul is a command-line tool and library that helps track code coverage in JavaScript projects. It analyzes which parts of your code have been executed, providing insights into testing effectiveness. This allows developers to identify untested areas, improving code quality and reducing potential bugs. Istanbul supports various coverage metrics like line, branch, and function coverage, and generates detailed reports in formats like HTML, LCOV, and Cobertura, facilitating seamless integration with different CI/CD pipelines.