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, so when you open a URL in a web browser, your laptop or computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address should be retrieved. With this a web 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 requested from the right location, a mail relay server discovers which server deals with the e-mails for the domain (MX record) so a message can be forwarded to the right mailbox, and so forth. Any modification of these sub-records is performed using the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for example. Each domain address has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.