The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain, so when you open a URL inside a browser, your laptop or computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. This way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an Internet protocol address and the website content is required from the correct location, a mail relay server finds out which server takes care of the e-mails for the domain address (MX record) so that a message can be sent to the needed mailbox, etc. Any change of these sub-records is conducted with the help of the company whose name servers are used, so you're able to keep the website hosting and switch only your email provider for example. Each and every domain has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.