The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Essentially, the zone is the selection 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 name is hosted and from which servers the DNS records for the domain name must 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 website content is required from the correct location, a mail relay server finds out which server handles the emails for the domain name (MX record) to ensure that a message can be forwarded to the needed mailbox, and so on. Any change of these sub-records is conducted with the help of the company whose name servers are used, permitting you to keep the web hosting and change only your email provider for example. Every single domain address has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.