The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain name, so when you open a URL within a 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 name 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 proper location, a mail relay server detects which server manages the emails for the domain name (MX record) to ensure 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 employed, allowing you to keep the website hosting and switch only your email provider for example. Every domain has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.