What is ......

MERN is a JavaScriptweb app stack (MongoDB/Express/React/Node.js)

It’s made up of Open Source components to provide an end-to-end framework for building dynamic web applications.

The MERN stack allows developers to use Javascript to implement the front end experience as well as the application logic and access the database.

From TOP to BOTTOM:

React.js - A JavaScript library for building interactive user interfaces

Express - Backend web application framework which runs on top of Node.js

Node.js - JavaScript runtime environment so you can implement backend JavaScript

Mongodb - Document database (noSQL) used by Node.js (in this case) to stores data as JSON (JavaScript Object Notation)

Resources:

https://www.mongodb.com/blog/post/the-modern-application-stack-part-1-introducing-the-mean-stack