Seen a few people recommending to switch your DNS to OpenDNS to avoid DNS filtering. OpenDNS is Cisco and is a logging DNS (meaning it store your query), maybe look for alternative like OpenNIC if you want to avoid big tech.

For the non technical, DNS is the system that convert domain name (like google.com) to an IP address usable by your device, like a phone book.

It Isnt encrypted, so there's no way to know if the server that respond is your selected target, and it can be snooped on/modified by anyone on the way to your target (aka ISP, bad actors).

DNSSEC try to fix this but only allow to confirm the response come from the good target, the query and response are still sent in clear. If you want full encryption you need something like DNSCrypt which is less trivial to set up and need to be supported by the server you use.

More so if you are using a custom DNS and it goes down, it will look like you have lost your internet connection (direct ip still works but most website block it)