The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain address, so when you open a URL within a browser, your PC asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. In this 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 address and the site content is requested from the correct location, a mail relay server finds out which server deals with the emails for the domain name (MX record) so a message can be forwarded to the appropriate mailbox, etc. Any change of these sub-records is performed with the help of the company whose name servers are employed, so that you can keep the website hosting and switch only your email provider for example. Each domain has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.