The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain address, so when you open a URL inside a browser, your computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain address ought to be retrieved. That way a 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 website content is required from the right location, a mail relay server detects which server manages the emails for the domain (MX record) so a message can be forwarded to the correct mailbox, etc. Any change of these sub-records is done with the help of the company whose name servers are employed, so you can keep the website hosting and switch only your email provider for example. Each domain has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.