What's an NS Record? - DNSimple Help
https://support.dnsimple.com/articles/ns-recordWhat’s an NS Record? An NS record delegates a subdomain to a set of name servers. Whenever you delegate a domain to DNSimple, the TLD authorities place NS records for your domain in the TLD name servers pointing to us. For example, there are the following entries delegating dnsimple.com to our name servers in the .com name servers:
Name server - Wikipedia
https://en.wikipedia.org/wiki/Name_serverThe Internet maintains two principal namespaces: the domain name hierarchy and the IP address system. The Domain Name System maintains the domain namespace and provides translation services between these two namespaces. Internet name servers implement the Domain Name System. The top hierarchy of the Domain Name System is served by the root name servers maintained by delegation by the Internet Corporation for Assigned Names and Numbers(ICANN)…