Any time one file depends on another, webpack treats this as a dependency. This allows webpack to take non-code assets, such as images or web fonts, and also provide them as dependencies for your application.
When webpack processes your application, it starts from a list of modules defined on the command line or in its config file. Starting from these entry points, webpack recursively builds a dependency graph that includes every module your application needs, then bundles all of those modules into a small number of bundles - often, just one - to be loaded by the browser.
Bundling your application is especially powerful for HTTP/1.1 clients, as it minimizes the number of times your app has to wait while the browser starts a new request. For HTTP/2, you can also use Code Splitting to achieve best results.