The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain address, so when you open a URL within a web browser, your personal computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain should be retrieved. With this a web 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 web site content is required from the correct location, a mail relay server detects which server handles the e-mails for the domain address (MX record) so a message can be delivered to the appropriate mailbox, and so forth. Any change of these sub-records is done using the company whose name servers are employed, so you're able to keep the web hosting and switch only your email provider for instance. Each domain has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.