What are animation nodes? So, I've come across this term while using other software as well. However, with respect to Blender, what are they? How does one use/access them and if possible could a simple animation be demonstrated using this?
It is an addon that adds a procedural node based modelling and animation tools and workflow to Blender.

You can learn more about it at the official GitHub repository

From the official description


  Node based visual scripting system designed for motion graphics in Blender.