The NS (Name Server) records of a domain name point out 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 globally where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. In this way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the website content is required from the proper location, a mail relay server discovers which server manages the emails for the domain name (MX record) so that a message can be delivered to the appropriate mailbox, etc. Any change of these sub-records is conducted with the help of the company whose name servers are employed, so you can keep the website hosting and change only your email provider for instance. Every domain has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.