Karma-commonjs is a Karma plugin that enables testing of CommonJS modules in the browser by providing a lightweight wrapper around your code and supporting Node's `require` algorithm. It's designed for developers who need to test CommonJS modules without using Browserify, offering faster builds and cleaner stack traces compared to Browserify-based solutions. The plugin requires explicit file specification in the Karma config and can be configured with a custom modules root folder for locating required modules.