The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain address, so when you open a URL inside a web browser, your PC asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name should be retrieved. This way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the website content is required from the proper location, a mail relay server discovers which server manages the e-mails for the domain address (MX record) to ensure that a message can be forwarded to the correct mailbox, and so forth. Any change of these sub-records is performed through the company whose name servers are used, permitting you to keep the website hosting and change only your email provider for example. Each domain name has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.