This record type is used to share the list of authoritative DNS servers for the domain. There are listed the names of the servers (ie domain names), not IP addresses. When you want to connect the device with the authoritative DNS server of some domain, at first find out all type of NS records for the domain at the higher-level DNS server, choose one of them and find the IP address for him, only then it can lead to a communication connection via the DNS protocol.
The same records are also stored directly in the records of the domain. To illustrate, this is the result of a request on the domain “cuni.cz” if you ask the server “ns.tld.cz”.
cuni.cz. 18000 IN NS ns.ces.net. cuni.cz. 18000 IN NS golias.ruk.cuni.cz.
We can see that we have received as response two NS records, which gives the desired result to us. We get the same response as such after asking “mff.cuni.cz” because the whole server does not know the answer, but it refers us to other DNS servers that it should know. Important is that we get the same result if we directly ask one of its authoritative DNS servers. This means that each domain has records stored own NS records to itself.