This is by far one of the most useful things i have installed on my Synology. Pi-hole is a DNS sinkhole, it blocks adverts and tracking sites at a domain level, and works across your entire network to protect all your devices.
You specify which domains to block via lists, which are freely available and generally fall into specific categories such as tracking, malware, ads etc..
Once setup with some ad-lists, you simply redirect all DNS queries on your home router to the internal IP address of the Pi-hole, Pi-hole then passes any DNS queries it receives to the upstream DNS servers which you specify (Adguard DNS, Cloudflare, QUAD9, Google etc), and then filters the response against the specified ad-lists.
The most significant benefit of using Pi-hole is that it protects all your connected devices at the network level, and negates the need to have ad-blockers installed on each individual device. It even protects devices like your TV, which otherwise wouldn’t have any defence against ads.
Pi-hole is really easy to setup on a Synology NAS using Docker, and i would highly recommend you check it out for yourself. There are already plenty of really good guides available for this so i’m not going to make another one, instead i’m going to point you to this guide which i think sums up the steps required very succinctly.