The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain, so when you open a URL inside an Internet browser, your laptop or computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address must be retrieved. That way a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the web site content is required from the proper location, a mail relay server finds out which server deals with the emails for the domain address (MX record) so a message can be forwarded to the appropriate 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 change only your email provider for instance. Each and every domain address has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.