The Name Servers of a domain reveal the DNS servers that are responsible for its DNS records. The IP of the site (A record), the mail server that manages the e-mails for a domain (MX records), any text record in free form (TXT record), pointing (CNAME record) etc are obtained from the DNS servers of the web hosting company and for any domain to be using them and to be pointed to their hosting platform, it has to have their name servers, or NS records. If you would like to open a site, for example, and you type the URL, the browser connects to a DNS server, which keeps the NS records for the domain name and the request is then redirected to the DNS servers of the hosting company where the A record of the web site is retrieved, allowing you to view the content from the correct location. Commonly a domain address has a couple of name servers that start with NS or DNS as a prefix and the difference between the two is only visual.