The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain name, so when you open a URL inside a web browser, your personal computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain name must be retrieved. In this way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the website content is requested from the correct location, a mail relay server detects which server manages the e-mails for the domain (MX record) to ensure a message can be forwarded to the needed mailbox, etc. Any change of these sub-records is done with the help of the company whose name servers are used, so that you can keep the web hosting and switch 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.