Express 24,430 occupies the middle of the stack. MongoDB 1,815 holds the data, React 7,897 draws the screen, and Express turns requests from the browser into database operations and back into JSON. It is the part of a MERN application you fully control: you choose the URLs, the status codes, the payload shapes and the security rules, and the layers on either side are written against those choices. This section positions Express against the raw http server of Networking and HTTP and lays out the project you will build.