The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain address, so when you open a URL within a browser, your PC asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain name must be retrieved. 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 site content is required from the proper location, a mail relay server detects which server deals with the e-mails for the domain (MX record) to ensure a message can be sent to the needed mailbox, etc. Any modification of these sub-records is done 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 domain name has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.