Official Guide - https://webpack.js.org/guides/hot-module-replacement/
Hot Module Replacement (or HMR) is one of the most useful features offered by webpack. It allows all kinds of modules to be updated at runtime without the need for a full refresh.
Official concept - https://webpack.js.org/concepts/hot-module-replacement/
Hot Module Replacement (HMR) exchanges, adds, or removes modules while an application is running, without a full reload. This can significantly speed up development in a few ways:
Retain application state which is lost during a full reload. Save valuable development time by only updating what's changed. Tweak styling faster -- almost comparable to changing styles in the browser's debugger.