Pi-hole is software that runs typically but not necessarily on a raspberry pi. It maintains a list of known advertising and tracking servers and blocks them by rerouting at the DNS level. For example an embed in a page tells your computer to contact tracking.facebook.com pihole tells your computer that that website is at 0.0.0.0 instead of it’s real IP address. Nifty thing is that you can redirect all of your DNS queries at the router so even devices that can’t normally run ad blockers can take advantage of it.