Registrar versus DNS host
Buying a domain and hosting its DNS are separate jobs. At the registrar you set which nameservers the parent zone (the TLD) should delegate to. Those nameservers then publish A, MX, TXT, and the rest. NS records are how that delegation is expressed in DNS.
A common outage: you change records at Cloudflare (or Route 53, or cPanel) while the registrar still delegates to the old host. The internet never sees your new records. This lookup shows who is actually authoritative from the resolver’s point of view.
What “good” looks like
At least two nameservers, ideally on separate networks. Hostnames that themselves resolve. Glue records at the parent if the NS names are inside the same zone (ns1.example.com for example.com). More in nameservers explained.