The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain address, so when you open a URL inside an Internet browser, your personal computer asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. That way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP address and the website content is requested from the proper location, a mail relay server discovers which server takes care of the e-mails for the domain address (MX record) so a message can be delivered to the right mailbox, and so forth. Any modification of these sub-records is done 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 instance. Every domain name has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.