The NS (Name Server) records of a domain name show 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 web browser, your PC asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain ought to be retrieved. This way a 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 web site content is required from the correct location, a mail relay server finds out which server handles the e-mails for the domain address (MX record) so that a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is done through the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for instance. Every single domain has a minimum of two NS records - primary and secondary, which start with a prefix like NS or DNS.