The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain, so when you open a URL within a web browser, your computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address ought to be retrieved. This way 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 website content is required from the proper location, a mail relay server detects which server takes care of the emails for the domain address (MX record) so a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is conducted through the company whose name servers are employed, so you're able to keep the website hosting and switch only your email provider for example. Each Internet domain has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.