The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain address, so when you open a URL in a web browser, your PC asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address should be retrieved. This way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the web site content is requested from the right location, a mail relay server finds out which server manages the e-mails for the domain name (MX record) to ensure that a message can be sent to the right mailbox, and so forth. Any change of these sub-records is conducted using the company whose name servers are employed, so you can keep the website hosting and change only your email provider for example. Every single domain has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.