The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain name, so when you open a URL in a browser, your computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain address ought to be retrieved. That way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the web site content is requested from the right location, a mail relay server finds out which server handles the e-mails for the domain (MX record) so a message can be sent to the appropriate mailbox, and so on. Any change of these sub-records is conducted through the company whose name servers are employed, permitting you to keep the website hosting and switch only your email provider for example. Each and every domain address has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.