The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain address, so when you open a URL within a browser, your PC asks the DNS servers around the globe 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 is so that the latter is mapped to an IP address and the site content is required from the proper location, a mail relay server finds out which server manages the e-mails for the domain address (MX record) to ensure a message can be delivered to the appropriate mailbox, and so on. Any change of these sub-records is conducted with the help of the company whose name servers are employed, enabling you to keep the web hosting and switch only your email provider for instance. Every domain name has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.