What is the MERN stack and how does it work together?

MERN is a full-stack JavaScript framework combining 4 technologies:

**M — MongoDB:** NoSQL database (stores data as JSON documents)
**E — Express.js:** Backend web framework for Node.js (handles routing, middleware)
**R — React:** Frontend library (builds the UI)
**N — Node.js:** JavaScript runtime for the backend

**How data flows:**
```
Browser (React)
↕ HTTP/REST API
Express + Node.js (Backend)
↕ Mongoose ODM
MongoDB (Database)
```

**Simple example:**
1. User fills a form in React
2. React sends POST request to Express API
3. Express validates and saves to MongoDB
4. MongoDB returns saved document
5. Express sends response to React
6. React updates the UI

**Why MERN is popular:**
- One language (JavaScript) for entire stack
- Fast development
- Large ecosystem
- High demand in startups