Raspberry pi is a small chip, single board computer about the size of a credit card. Developers are able to buy the small computer for around $35 and build a wide range electronics with it.
The small chip involves some more advanced set up than other home automation hardware. But for tech savvy users it can provide almost limitless solutions for customized home automation and iOT devices. It can be used to create specific hardware devices, or to integrate many different ones together in your smart home.
Aside from programming your own interactions, raspberry Pi allows you to download and install other software onto it. One amazing example of this is using a free software package called magic mirror. This allows you to create a smart mirror using your own hardware, and install the software on a rasberry pi behind the mirror. There is also plenty of options among the open source community for many of these tools, usually found on GitHub (ex: https://github.com/evancohen/smart-mirror).
Building a custom LED strip:
Demo of an advanced smart home setup with raspberry pi, and other home automation products (home assistant, phillips hue, and z-wave):
A good collection of raspberry pi articles about home automation (from the raspberry pi website):