What nameservers do
Nameservers are the servers that hold the DNS records of a domain. When someone looks up your website or sends you mail, their resolver first finds out which nameservers are responsible for your domain and then asks one of them. Whoever runs those nameservers decides what every other record says, which is why the NS set is the first thing to check when DNS behaves unexpectedly.
The NS records usually reveal the DNS provider at a glance: a registrar, a CDN such as Cloudflare, a cloud provider or servers the owner runs.
Two places, one answer
A domain has nameservers listed in two places. The registry holds the delegation, which you set at your registrar. The zone itself also contains NS records, served by your DNS provider. Resolvers follow the delegation to find your zone, so that is the set that actually routes traffic, but both should list the same servers.
OrbitProbe reads the delegation from the registration data and the NS records from DNS, then tells you whether they match. A mismatch is expected for a short while during a provider change. At any other time it usually means a change was made in one place and forgotten in the other.
Subdomains
A subdomain such as www or mail normally has no NS records of its own and uses the nameservers of the parent domain. It only has its own NS records when it has been delegated to different servers, which the lookup will show.