The Domain name system (DNS) is a system that translate your friendly domain name into IP address. This is not a server or software but more massive network of servers that comprises largest digital database in a world. Each server in this network is called name server (NS).
In the beginning, before world wide web, computers would connect to each other via IP addresses, which are more like strings of segmented numbers separated by dots. DNS attach your domain name to IP address so that your website visitors can easily remember and come back later on to the web addresses they have visited earlier.
As an example 5.150.255.234 is IP address for ST Registry WHOIS Server and domain whois.nic.st refer to this IP address.