Ted:
Don't be worried. You can always go back and check the box that says do it automatically.
DNS = Domain Name Server
ISPs (Internet Service Providers) own them, and they use them to tell your computer the IP address of a website you put in - like
www.therx.com = 69.20.60.150. I think thats how they work.
Comcast's suck right now so use the ones they mentioned above until it's fixed (4.2.2.1, 4.2.2.2), or, hell, just keep using those. They are owned by Verizon, and they are faster anyway.