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 in an Internet browser, your PC asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. With this 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 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) to ensure that a message can be forwarded to the needed mailbox, and so forth. Any change of these sub-records is done with the help of the company whose name servers are used, so you're able to keep the website hosting and change only your email provider for example. Every single domain has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.