The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain name, so when you open a URL in an Internet browser, your personal computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. That way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an Internet protocol address and the web site content is required from the correct location, a mail relay server discovers which server handles the e-mails for the domain (MX record) so a message can be forwarded to the right mailbox, and so on. Any modification of these sub-records is conducted with the help of the company whose name servers are used, so that you can keep the website hosting and change only your email provider for example. Every single domain address has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.