Any modern programming language has a package manager for managing external dependencies in your program. In simple terms, it means downloading and using various external libraries/frameworks in your program; for example, Clojure has Leiningen (available at http://leiningen.org/), Ruby has RubyGems (available at https://rubygems.org/), and .NET has Nuget (available at https://www.nuget.org/). NPM (available at https://www.npmjs.com/) is the official package manager for Node.js. NPM is bundled with Node.js these days, so as soon as you install Node.js (for your OS), NPM is ready to serve you.

You might think that you are just developing an Angular application and you don't plan on writing server-side JavaScript (or using Node.js), so why do you even need NPM? The answer to your quandary is that NPM is required to install the other tools mentioned in the next sections, and it's needless to say that you'll thank yourself for installing Node.js and NPM.