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 inside a web browser, your laptop or computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address ought to be retrieved. With this a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an Internet protocol address and the website content is required from the correct location, a mail relay server detects which server takes care of the e-mails for the domain name (MX record) to ensure a message can be delivered to the appropriate mailbox, and so forth. Any change of these sub-records is performed using 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 has at least 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.