Something that's unique to web application development is that it's mostly static content that's loaded into the browser. The browser loads the HTML, followed by any scripts which are then run to completion. There's a long-running process that continuously refreshes the page based on the state of the application—everything is over a network.

As you can imagine, this is especially annoying during development when you want the see the results of your code changes as they're introduced. You don't want to have to manually refresh the page every time you do something. This is where hot module replacement comes into play. Essentially, HMR is a tool that listens for code changes, and when it detects one, it sends a new version of the module to the browser:

Even with a tool like Webpack and its HMR component, it's time-consuming and error-prone to get this setup working correctly, even for simple React projects. Thankfully, there's tooling that hides these setup details from developers today.