MEAN comprises of a schema-less document-oriented database-Mongo DB, a server-side JavaScript framework-ExpressJS, a front-end web application framework–AngularJS/AngularJS 2.0 and a server-side JavaScript runtime environment-NodeJS, whereas the MERN has the replacement of AngularJS/AngularJS 2.0 with a JavaScript library-ReactJS. All of the techies here can independently exist, but when used in combination, the developer can make use of their amazing capabilities and implementations in unison.
Initially, the developer was bound to have a solid grasp on the conceptual implementation of multiple languages for different tiers of the application, but gradually, with the introduction of these powerful stacks, the work simplified. Now the developers need to focus on only one language, the JavaScript. This type of approach provided a prominent enhancement in the manner the application tiers sent and received data in the form of JSON format.
And an interesting fact is that the whole of this stack thing is developed in JavaScript!