`webpack-isomorphic-tools` is a deprecated utility for enabling server-side rendering of Webpack-built applications, particularly those using React. It allows `require()` calls to work on the server for assets like images and CSS by providing a fallback mechanism when Node.js cannot natively handle these Webpack-specific imports. It's primarily intended for legacy projects still using this approach, as modern alternatives like `universal-webpack` or frameworks like Next.js are now recommended.