- 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.