The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain name, so when you open a URL within a browser, your computer asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain name must be retrieved. With this a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the website content is requested from the proper location, a mail relay server finds out which server takes care of the e-mails for the domain (MX record) to ensure a message can be sent to the right mailbox, and so on. Any modification of these sub-records is conducted using the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for instance. Each domain address has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.