The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain, so when you open a URL within a browser, your laptop or computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain name must be retrieved. In 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 IP and the website content is required from the right location, a mail relay server discovers which server deals with the emails for the domain name (MX record) to ensure that a message can be forwarded to the needed mailbox, etc. Any change of these sub-records is performed through the company whose name servers are used, permitting you to keep the web 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 like NS or DNS.