It's a network-wide ad blocker. In layman's terms, a "Pi-hole" is a device connected on a network that blocks ads on devices that are connected to it. It's usually a Raspberry Pi and it's a "black hole" where adverts get sucked in, hence "Pi-hole."
Still don't know what a Pi-hole is? Learn more about it here: https://pi-hole.net/
This repository contains all the configs and settings I have for my Pi-hole. This contains all my block lists as well as my blacklist and whitelist. That's all there is to it.