I have changed mine to opendns. They have a distributed network on dns servers and offer filtering protection for different categories.
Dns is like a phone book for computers. They change text phrases humans use (ie CNN.com, dal reports.com, etc.) into machine readable ip addresses (ie 220.127.116.11 , 18.104.22.168). When a dns server goes down, as some isp servers have done in the past, the Internet appears to not work. Actually the "phone book" the computer use is actually not working, if the user actually typed in the IP address, the website would likely work but not well.
I use this product to find a fast and local dns server, »www.grc.com/dns/benchmark.htm . It looks at the various available dns servers then compares features and speed.
How to change dns server. You can change the dns server you use at the router or per computer/device. The router is usually easier.
Chilhowee, MO (2.7 miles from town via copper path)
Centurylink 4.0M (actual 2.0-2.5M) w/Local and LD phone $70.00