The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain address, so when you open a URL inside a browser, your personal computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain name should be retrieved. 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 site content is required from the proper location, a mail relay server detects which server manages the e-mails for the domain address (MX record) to ensure a message can be sent to the needed mailbox, and so forth. Any modification of these sub-records is conducted with the help of the company whose name servers are used, so that you can keep the website hosting and switch only your email provider for instance. Every domain name has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.