The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain, so when you open a URL in 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 address must be retrieved. That 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 required from the right location, a mail relay server finds out which server takes care of the emails for the domain (MX record) so a message can be delivered to the appropriate mailbox, etc. Any change of these sub-records is conducted through the company whose name servers are employed, so you're able to keep the web hosting and change only your email provider for instance. Each and every domain address has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.