The Name Servers of a domain reveal the DNS servers that are responsible for its DNS records. The IP of the website (A record), the mail server that manages the e-mails 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 hosting provider and for any domain address to be using them and to be directed to their hosting platform, it has to have their name servers, or NS records. If you would like to open a website, for example, and you insert the URL, the web browser connects to a DNS server, which keeps the NS records for the domain address and the request is then redirected to the DNS servers of the webhosting provider where the A record of the website is retrieved, enabling you to view the content from the proper location. Usually a domain has two name servers that start with NS or DNS as a prefix and the contrast between the two is just visual.