The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain name, so when you open a URL in an Internet browser, your laptop or computer asks the DNS servers worldwide where the domain address 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 is so that the latter is mapped to an IP address and the site content is requested from the proper location, a mail relay server finds out which server handles the e-mails for the domain name (MX record) so a message can be delivered to the appropriate mailbox, and so on. Any modification of these sub-records is performed using the company whose name servers are employed, allowing you to keep the website hosting and change only your email provider for example. Every Internet domain has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.