Answers for "what does npm install do?"

npm install

npm install -g sass

install npm

curl -sL https://deb.nodesource.com/setup_6.x | sudo bash -
curl -sL https://deb.nodesource.com/setup_10.x | sudo -E bash -
sudo apt-get install -y nodejs
sudo apt-get install -y npm

install an npm package

npm install package-name

To save as a dependency:
npm install package-name --save

what is npm install

npm install module-name
npm i module-name
puts module-name package (js library file) 
into node_modules folder
so you can require it in node app