DNS is for converting website name to ip addresses, like phone book in your phone, instead of remembering ip address, you remember website name, same as you dont need to remember phone number, just name of a person. you can use any DNS available to you, google offer free DNS service 8.8.8.8 and 8.8.4.4, or u can use the 1 offer by your ISP, technically the 1 offer by your ISP should be slightly faster, cause its closer to you, but gov has the right to ask ISP to ban certain website using the DNS(stupid way, but work for some ppl who dont know how to use other DNS), i dont see a real downside not to use google DNS as i can assume its more reliable than the 1 offer by ISP.