The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain address, so when you open a URL within a web browser, your PC asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain name should be retrieved. In this way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an Internet protocol address and the website content is requested from the right location, a mail relay server discovers which server deals with the e-mails for the domain name (MX record) so a message can be forwarded to the correct mailbox, and so forth. Any modification of these sub-records is done through the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for instance. Every domain has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.