What are nameservers?
Nameservers are the servers that hold a domain’s DNS records and answer for them. The registry for the domain’s extension publishes NS records pointing at them, which is how every resolver finds them. Changing nameservers moves the whole zone, mail records included, to whichever servers you name.
Reading the result
The nameservers are listed with their TTLs and the DNS provider their names belong to, when recognised. Each one is checked for an address, since a resolver cannot ask a nameserver it cannot reach. When resolvers return SERVFAIL, the tool asks again with DNSSEC validation off, which tells a broken signature apart from servers that do not answer.
Beat runs on the same engine, on a schedule, across all 41 checks and 25 blacklists. What it adds is asking again, noticing when an answer changes and telling you what to do about it.
Common problems it finds
Only one nameserver
RFC 1034 requires every zone to be served by at least two, and RFC 2182 advises more. With one, the domain and its mail vanish whenever that server is down.
A nameserver with no address
A name in the NS set that does not resolve is one resolvers can never ask. They time out on it and try the others, which slows every lookup.
DNSSEC that fails after a move
Moving to new nameservers while the old DS record stays at the registrar can break validation, and validating resolvers then treat the domain as unreachable.
Nameservers from two providers
Leftovers from a move mean resolvers sometimes ask the old provider, which answers with old records. Keep both only if both serve the same zone on purpose.