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, so when you open a URL within a web browser, your personal computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain must be retrieved. This 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 web site content is required from the proper location, a mail relay server detects which server deals with the emails for the domain name (MX record) to ensure that a message can be forwarded to the appropriate mailbox, and so forth. Any change of these sub-records is conducted with the help of the company whose name servers are used, so you're able to keep the website hosting and switch only your email provider for instance. Every single domain name has at least two NS records - primary and secondary, that start with a prefix such as NS or DNS.