The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain, so when you open a URL inside a 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 name must be retrieved. In this way 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 and the website content is required from the correct location, a mail relay server finds out which server deals with the e-mails for the domain (MX record) so a message can be sent to the needed mailbox, and so on. Any change of these sub-records is performed through the company whose name servers are used, so you're able to keep the web hosting and change only your email provider for example. Every single domain has at least two NS records - primary and secondary, that start with a prefix such as NS or DNS.