The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Essentially, the zone is the selection 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 name is hosted and from which servers the DNS records for the domain name should 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 requested from the right location, a mail relay server finds out which server takes care of the e-mails for the domain name (MX record) to ensure a message can be sent to the appropriate mailbox, and so forth. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, allowing you to keep the website hosting and switch only your email provider for instance. Every single domain has at least two NS records - primary and secondary, that start with a prefix such as NS or DNS.