It is, according to their github page, "a lightweight 3D library with a very low level of complexity — in other words, for dummies."
It's also extremely vast, though, and there are all kinds of amazing projects that can be made.
Their github page: https://github.com/mrdoob/three.js/
Install with npm install so that you'll be able to run grunt serve to see
your images on local host!
It is a library that's meant to make rendering 3D images to browsers easier. It works with rendering APIs such as WebGL, except using three.js means much, much less code!
But since it's tool for graphics, let's actually look at some examples (selected from threejs.org): http://news-lab-trends-experiment.appspot.com/ http://www.dilladimension.com/ http://middle-earth.thehobbit.com/
Fair warning: it's not going to look 1/100th as cool as the examples I just showed you, but it's a way to take a look at the most basic elements of a three.js script.