Even though there are plenty of good answers out there, the one that I like the most is this one, Node Js is an environment to run JavaScript outside the browser.
NodeJs was created in 2009 and It is built on Chrome's JavaScript V8 engine.
As you probably are aware, every browser has an engine, a tool that compiles our code down to machine code and Chrome uses one by the name of V8.
In extremely simple terms, the creator of Node Ryan Dahl ripped out the V8 engine and built Node on top of it.
Since the moment it was created, Node has evolved tremendously and while there are many things to like about Node, I particularly enjoy the large community around Node since that exceedingly saves time on feature development, as well as the fact that with the help of Node, it has never been easier to build full-stack apps since both front-end and back-end are essentially built in the same language, and you guessed it, that language is our beloved JavaScript.