NodeJs

Node is an execution environment for Javascript, people often confuse it with express, which is only one of the possible serverside frameworks that are built on top of node. Node can be used to make GUI installable apps, and command line which has nothing to do with web and APIs also, but making APIs is generally the primary use case where developers choose node.

npm stands for Node Package Manager which is a tool used to manage dependencies just like maven or Gradle for java or pip for python.

Node always runs in a single process and thread by default. The way it's able to manage simultaneous connections is via something called Event Loop.

This video goes over some basics of nodeJs

You will not be able to fully appreciate the event loop till you've dived a little more into ES6 language features, so make sure you've completed that section first. This video does a really good job of explaining the event loop.