CSS Modules allows us to write CSS rules that are restricted to a certain part of the UI. It is not an official specification or browser implementation, but rather a process enabled by a build tool like Webpack. In contrast to a regular CSS selector, which can match any element, a CSS Modules selector can only match elements in a specific part of the UI (e.g. in a specific React Component). Build tools accomplish this by transforming the selectors we write into new, globally unique selectors and then applying the new selector to the associated UI element.
Classic DEV Post from Aug 30