Webpack is a popular open-source tool and module bundler application for JavaScript applications. It is commonly used in web development projects to merge multiple JavaScript files and their dependencies into a single JavaScript bundle that can then be used in web applications.

Webpack provides a powerful and flexible way to manage and optimize various resources such as JavaScript, CSS, images, and other file types. It also enables the use of features like code splitting and tree shaking to optimize the size of the generated bundles and improve the loading times of web applications.

Essentially, Webpack works by creating a "dependency graph" that tracks all modules and their dependencies within an application. Based on this graph, Webpack can then analyze, optimize, and concatenate the required resources into one or more bundles that can be loaded by the browser.

Webpack has become one of the most popular build tools in web development and is often used in conjunction with other tools and frameworks such as React, Vue.js, and Angular to create modern web applications.