The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain, so when you open a URL in a browser, your personal computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. That 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 web site content is requested from the proper location, a mail relay server finds out which server manages the e-mails for the domain address (MX record) so that a message can be sent to the needed mailbox, etc. Any modification of these sub-records is conducted through the company whose name servers are employed, allowing you to keep the website hosting and switch only your email provider for example. Every single domain has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.