The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain address, so when you open a URL in a browser, your laptop or computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain should be retrieved. With this a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an Internet protocol address and the web site content is required from the right location, a mail relay server discovers which server handles the emails for the domain (MX record) to ensure that a message can be sent to the appropriate mailbox, and so on. Any modification of these sub-records is performed using the company whose name servers are used, allowing you to keep the website hosting and switch only your email provider for example. Every domain has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.