The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain name, so when you open a URL inside an Internet browser, your laptop or computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address should be retrieved. That 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 address and the site content is required from the correct location, a mail relay server detects which server manages the e-mails for the domain name (MX record) to ensure a message can be delivered to the appropriate mailbox, and so on. Any modification of these sub-records is done using the company whose name servers are used, so you're able to keep the web hosting and switch only your email provider for instance. Every domain address has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.