A DNS (domain name system) server is like a 411 service for computers. A client computer sends a request that includes a domain name (such as www.google.com), and the server returns its IP address, such as 66.102.7.104.
While a DNS server is not strictly required by TCP/IP communication, it is required if domain names will be used. This means that, practically, all servers needs to have the DNS serves set up correctly. Otherwise, all DNS related operations cannot be done.