The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain address, so when you open a URL inside an Internet browser, your computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain name should 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 IP address and the site content is required from the correct location, a mail relay server finds out which server handles the e-mails for the domain name (MX record) so that a message can be sent to the appropriate mailbox, etc. Any change of these sub-records is performed using the company whose name servers are employed, so you're able to keep the website hosting and switch only your email provider for instance. Every single domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.