The DNS Server
One of the functions of the Domain Controller is that network names and the IP addresses are resolved properly. This is like a phone book.
A phone book will have three columns, the name, the address, and the phone number (in the white pages). To find a phone number you look up the name first and then the phone number will be on the same row. In computer operations this is called name resolution.
In a computer network, communication occurs via an IP address. This is a 32 bit binary number that is broken up into four octets. For example, 192.168.15.200 is an IP number. If we convert that to binary it becomes:11000000. 10101000. 00001111. 11001000.
That is not easy to remember, but the number in Dec is easier to remember, but even easier is the name, which we can say is DC-1. With name resolution we can associate an IP address with a computer. That is what the DNS server routinely does.
The other job the DNS server does is point a computer toward the Internet, or to a computer inside a network that point towards the Internet.
Once the DNS server is configured the next server that must be on a Domain Controller is a DHCP server.