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 address, so when you open a URL within a web 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 ought to be retrieved. In this way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the web site content is required from the proper location, a mail relay server finds out which server handles the e-mails for the domain (MX record) so that a message can be sent to the right mailbox, and so forth. Any modification of these sub-records is conducted through the company whose name servers are employed, so you can keep the website hosting and change only your email provider for example. Every single Internet domain has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.