Find the authoritative nameservers a domain is delegated to.
NS records at the parent zone delegate a domain to its authoritative nameservers. Whoever controls those servers controls all DNS answers for the domain — that is why NS checks matter during migrations.
google.com NS → ns1.google.com …Wrong NS after a migration or transfer means the whole domain stops resolving.
Usually 2–4 for redundancy across networks.
NS names the servers in charge of the domain; A gives the IP of a host.