The NS (Name Server) records of a domain point out 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 globally where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. This 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 site content is requested from the right location, a mail relay server discovers which server manages the e-mails for the domain address (MX record) so a message can be forwarded to the correct mailbox, and so on. Any modification of these sub-records is done through the company whose name servers are employed, so you're able to keep the web hosting and change only your email provider for instance. Each and every domain address has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.