The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain, so when you open a URL within an Internet browser, your computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. This way a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the web site content is required from the proper location, a mail relay server detects which server manages the e-mails for the domain (MX record) so a message can be delivered to the correct mailbox, etc. Any modification of these sub-records is performed through the company whose name servers are employed, so you can keep the website hosting and change only your email provider for example. Each and every domain has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.