The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain address, so when you open a URL in an Internet browser, your personal computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. With this a browser finds out what the A or AAAA record of the domain 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 detects which server manages the e-mails for the domain (MX record) so that a message can be forwarded to the needed mailbox, and so forth. Any modification of these sub-records is performed using the company whose name servers are used, so you're able to keep the web hosting and switch only your email provider for instance. Every domain name has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.