Instead of the client (the browser) resolving hundreds of dependencies and downloading them directly from the client, we would download all dependencies on the server, concatenate them in the right order into a single file (or bundle), and send that to the client. The bundle contains the application and all dependencies, and can be loaded like any regular script. Because all dependencies are resolved ahead of time, the time required to resolve dependencies on the client are eliminated.

But because everything is crammed into one file, the bundle may grow quite large, but the load time will be reduced as the client doesn't need to make hundreds of separate requests; now it's just one. Furthermore, if one of the external servers are down, it would not affect our bundled code, as this is served from our own server.

There are four module bundlers that you'll encounter in the wild: Browserify, Webpack, Rollup, and Parcel.