The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain, so when you open a URL within a 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 must be retrieved. With this a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an Internet protocol address and the site content is requested from the proper location, a mail relay server discovers which server deals with the e-mails for the domain (MX record) to ensure that a message can be forwarded to the needed mailbox, and so forth. Any modification of these sub-records is performed with the help of the company whose name servers are used, so you can keep the web hosting and change only your email provider for example. Every domain has no less than 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.