Dns is stand for (domain name system ) its a service used to convert the names into ip addresses , there is tow primary zones in the dns
1- forward luckup zone and its used to convert the name into ip addresses
2-reverse luckuo zone and the use of this zone is opposite of the above zone , to convert the ip address to name
the dns is a primary service that related to the active directory and the active directory can not work without the dns and there will be no network if the dns is not working .
when a client attempt to open a web site lets say www.exampl.com he sends a request to its local dns and if the local dns does't know the answer he will ask the root dns and the root dns will direct the dns to the high level domain (.net , .com , .org , .....) so the authoritative dns will send the ip address to ur local dns and the he will cache it and send it to u
this is a brief overview of how the DNS works
hope its helpful