React Router is used in web and native applications developed with React. The book assumes that you have a good understanding of JavaScript and some of the new language features introduced in ECMAScript 6, such as classes and spread operators.
The book provides a brief introduction to React and component-based architecture in React. Some of the other core concepts of React are documented at https://reactjs.org.
The book assumes that the reader has used Node.js and NPM to install libraries and packages from the NPM repository.