The Name Servers of a domain reveal the DNS servers that are responsible for its DNS records. The IP address of the web site (A record), the mail server that manages the e-mails for a domain name (MX records), any text record in free form (TXT record), directing (CNAME record) etc are taken from the DNS servers of the web hosting provider and for any domain address to be using them and to be directed to their hosting platform, it needs to have their name servers, or NS records. If you would like to open a site, for example, and you enter the URL, the browser connects to a DNS server, which keeps the NS records for the domain name and the request is then forwarded to the DNS servers of the webhosting provider where the A record of the site is obtained, so you can see the content from the correct location. Ordinarily 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.