The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain, so when you open a URL within an Internet browser, your laptop or computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address ought to be retrieved. In this way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an Internet protocol address and the site content is required from the right location, a mail relay server finds out which server handles the e-mails for the domain (MX record) to ensure a message can be sent to the needed mailbox, and so forth. Any change of these sub-records is conducted using the company whose name servers are used, so you can keep the website hosting and change only your email provider for instance. Every domain address has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.