The Name Servers of a domain reveal the DNS servers that handle its DNS records. The IP of the website (A record), the mail server that takes care of the e-mails for a domain name (MX records), any text record in free form (TXT record), pointing (CNAME record) etc are taken from the DNS servers of the hosting provider and for any domain name to be using them and to be forwarded to their hosting platform, it has to have their name servers, or NS records. If you would like to open a site, for instance, and you enter the URL, the browser connects to a DNS server, which keeps the NS records for the domain and the request is then pointed to the DNS servers of the webhosting provider where the A record of the site is obtained, so that you can view the content from the proper location. Usually a domain name has a couple of name servers that start with NS or DNS as a prefix and the difference between the two is just visual.