The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain name, so when you open a URL within a browser, your computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain address ought to 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 web site content is required from the proper location, a mail relay server finds out which server handles the emails for the domain (MX record) to ensure that a message can be sent to the correct mailbox, and so forth. Any change of these sub-records is performed through the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for example. Each domain name has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.