Node is a runtime engine for Javascript code. It utilizes the V8 Javascript runtime engine that is used in the Chrome web browser. It let's you run Javascript code on a machine as a standalone process without using a web browser. Node is written in C++.
Why Node? I'll let Node make it's own case for that here. As a C# developer I would probably solve similar problems using console applications or a Windows service. However, it's certainly good to be aware of other tools for solving problems. Also, NPM (node package manager) installs as part of Node and is an important tool in web development.
Go to nodejs.org to download and install Node. Download the long term servicing (LTS) version. Node can be uninstalled using Add/Remove programs. If you want to upgrade to a newer version of Node, I would recommend uninstalling the previous version first.
The install downloads a standard .msi package which opens an installation wizard. Select the following:
node --version
The version of Node you installed will be displayed (ex. v12.16.1)
Here is a really good free book for getting started with Node development: Learning Node.js with Azure
(note: the book doesn't require Azure, and can be following using Visual Studio Code locally)