The Name Servers of a domain reveal the DNS servers that are responsible for its DNS records. The IP address of the site (A record), the mail server that takes care of the emails for a domain (MX records), any text record in free form (TXT record), forwarding (CNAME record) etc are taken from the DNS servers of the website hosting company and for any domain name to be using them and to be forwarded to their hosting platform, it ought to have their name servers, or NS records. If you would like to open a site, for instance, and you input the URL, the web browser connects to a DNS server, which keeps the NS records for the domain address and the request is then sent to the DNS servers of the webhosting provider where the A record of the web site is retrieved, so you can view the content from the right location. Ordinarily a domain has two name servers that start with NS or DNS as a prefix and the distinction between the two is just visual.