NodeJS is a JavaScript runtime implementation. The JavaScript programming language (not to be confused with Java) is most well known for its use in web browsers. NodeJS makes it possible to also run JavaScript on servers and desktops. MountainLab-js is implemented in JavaScript and uses NodeJS as its runtime environment.
It is important that you install a recent version of NodeJS (it is a rapidly-evolving project). It is often not sufficient to simply use the default version installed by the operating system. Versions higher than 8 are generally working well.
npm is node package management software (the npm command) and an online package repository (https://npm.io). npm is to nodejs as pip is to python.
On Ubuntu 16.04, use the following:
curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
sudo apt-get install nodejs
For more information, visit nodejs.org.