The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain address, so when you open a URL inside an Internet browser, your personal computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. In 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 Internet protocol address and the website content is requested from the correct location, a mail relay server detects which server takes care of the e-mails for the domain (MX record) to ensure that a message can be forwarded to the appropriate mailbox, etc. Any change of these sub-records is done using the company whose name servers are employed, so you're able to keep the website hosting and change only your email provider for instance. Every single domain has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.