What does Express.js do in the MEAN stack?

You can think of Express as a utility belt for creating web applications with Node.js. It provides functions for pretty much everything you need to do to build a web server. If you were to write the same functionality with vanilla Node.js, you would have to write significantly more code. Here are a couple examples of what Express does:

• REST routes are made simple with things like
  • app.get('/user/:id', function(req, res){ /* req.params('id') is avail */ });
• A middleware system that allows you plug in different synchronous functions that do different things with a request or response, ie. authentication or adding properties
  • app.use(function(req,res,next){ req.timestamp = new Date(); next(); });
• Functions for parsing the body of POST requests
• Cross site scripting prevention tools
• Automatic HTTP header handling
  • app.get('/', function(req,res){ res.json({object: 'something'}); });

Generally speaking, Sinatra is to Ruby as Express is to Node.js. I know it's not a PHP example, but I don't know much about PHP frameworks.

• MongoDB = database
• Express.js = back-end web framework
• Angular = front-end framework
• Node = back-end platform / web framework

Basically, what Express does is that it enables you to easily create web applications by providing a slightly simpler interface for creating your request endpoints, handling cookies, etc. than vanilla Node. You could drop it out of the equation, but then you'd have to do a lot more work in whipping up your web-application. Node itself could do everything express is doing (express is implemented with node), but express just wraps it up in a nicer package.

I would compare Express to some PHP web framework in the stack you describe, something like slim.