This is meant for beginners as an exercise or learning project.
MERN is a full-stack web development framework, composed of four technologies MongoDB, ExpressJS, ReactJS, NodeJS
MongoDB: A document-oriented, No-SQL database used to store your app data.
NodeJS: It is a JavaScript runtime environment. It provides you with the ability to run JavaScript in a machine rather than in a browser.
ExpressJS: A framework layered on top of NodeJS, used to build the backend of a site using NodeJS functions and structures. Since NodeJS was not developed to make websites but rather run JavaScript on a machine, ExpressJS was developed.
ReactJS: It is a library created by Facebook. It is used to build UI components that create the user interface of the single page web application.
Mainly because this stack is completed composed utilizing the JavaScript language, most code you are writing from back to front is javaScript.