It took me a while to understand what 'stack' actually meant. When I realized that WAMP was actually a 'stack', I grasped it immediately.
I have been using WAMP for about a decade, so I understood that it was just a compilation of the software needed to produce projects with that software.
WAMP simply means Windows (OS) Apache (Server) MySQL (Database) and PHP (Language). So MEAN means MongoDB (Database) ExpressJS (serverside javascript) AngularJS (clientside javascript) Node JS (more javascript, like the server).

I downloaded Node JS, and MongoDB. For an IDE (integrated development environment) I downloaded and installed ATOM. Apparently ATOM comes with NodeJS built in, so I did not need to have downloaded or installed it beforehand.

I am currently wading through the tutorials I can find, for Express JS.