The Modern Application Stack - Part 5: Using ReactJS, ES6 & JSX to Build a UI (the rise of MERN) is about building a front-end client application using ReactJS, a JavaScript library for building interactive user interfaces. The application uses the MERN stack, which includes MongoDB as the database, Express.js as the backend server, and ReactJS as the frontend framework. The article explains how to build the ReactJS client application, including creating components, handling state changes, and making HTTP requests to the backend server. It also discusses the benefits of using ReactJS, such as its ease of use and flexibility. Additionally, it compares ReactJS with Angular 2, highlighting their differences in power and complexity. The article concludes by summarizing the MERN stack and mentioning future posts that will explore other technologies for building front-end applications.