The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the range 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 address should be retrieved. 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 and the site content is required from the proper location, a mail relay server finds out which server manages the e-mails for the domain address (MX record) to ensure that a message can be forwarded to the correct mailbox, etc. Any modification of these sub-records is done with the help of the company whose name servers are used, enabling you to keep the website hosting and change only your email provider for instance. Every domain address has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.