MERN Stack is an open-source JavaScript stack used for ease in the development and deployment of applications. 'MERN' stands for the four technologies that make up the stack: MongoDB, Express.js, React, and Node.js. MongoDB is a cross-platform document-oriented database used for JSON-like storage, including ad-hoc queries, indexing, replication, and load balancing. Express is a Node.js back-end web application framework utilized for building web applications and APIs. React is a front-end library used for making user interfaces. And lastly, Node.js is a back-end JavaScript runtime environment, which allows programmers to run code outside of a web browser. MERN stack is not to be confused with MEAN web dev stack, which uses Angular.js instead of React.js as the JavaScript library.

It is crucial to have a foundational understanding of web development before learning the MERN stack. So, HTML5, CSS3, and JavaScript skills are all required before you take on learning MERN Stack. In addition, it is good to have a working knowledge of Git and GitHub, the essentials of HTTP, and some understanding of JavaScript frameworks. Furthermore, it is also beneficial to be familiar with back-end language, database/web storage, and web application architecture. These skillsets will make for a smoother learning process as you navigate the intricacies of MERN Stack.

MERN Stack remains completely open-source, uses only one programming language (JavaScript), and has superior UI rendering and performance compared to competing Stacks. The only difference between MERN Stack and MEAN Stack is that the former uses React.js (instead of Angular.js) for the JavaScript library/framework. Thus, the framework and library are where the differences for UI rendering and performance come into play. React.js library is newer than the Angular.js framework, but beware when comparing the two because both have pros and cons. Also, some programmers have noted Angular.js's steep learning curve when compared to React.js.