What the parent says, what your nameservers say, and whether they agree.
Every other DNS check on this site asks a resolver a question and reads the answer. This one asks the machines that make the answer possible whether they agree with each other, which is a different job. It queries the nameservers of the parent zone directly, with recursion turned off, so what comes back is the referral itself rather than somebody's cached copy of it. Then it asks each of your own nameservers the same questions in turn and compares what they say.
That difference matters because the faults it finds are invisible to an ordinary lookup. A nameserver that is listed in the delegation but answers without the authoritative flag still answers, so a resolver that lands on it gets records that are not your zone's. A secondary that stopped transferring still replies, with a zone that is months old. A server that refuses TCP works perfectly until a record set grows past 512 bytes. In each case a normal lookup succeeds most of the time, and the failure is intermittent, which is the hardest kind to chase.
The DNSSEC check here is the one worth reading even if you are sure it is fine. It does not validate signatures. It recomputes the fingerprint your registrar published and asks whether it still matches a key your zone actually serves. Re-signing a zone without telling the registrar produces exactly that mismatch, and it fails closed: every validating resolver refuses the domain, its mail and its website together, while the zone itself looks perfectly healthy to anyone checking it without validation.
Glue is the addresses of your nameservers, published by the parent zone alongside the delegation. You need it when a nameserver is named inside the zone it serves: finding ns1.example.com's address means asking example.com's nameservers, which is ns1.example.com, and the loop only breaks if the parent supplies the address. A nameserver named outside the zone, like one at a DNS provider, needs no glue, and tools that report its absence as a fault are wrong.
Usually not. Anycast providers announce their nameserver addresses from hundreds of locations under a single AS number, and that is generally more resilient than two servers you run yourself. This page says so rather than warning about it. What it does warn about is nameservers sharing a single /24, because that normally means one rack and one upstream.
It answers, so nothing looks down. But the answer does not carry the authoritative flag, which means that server was delegated a zone it was never configured to serve. Resolvers pick nameservers more or less at random, so a share of your lookups get an answer that is not your zone's, and which share depends on which resolver the visitor uses. That is why it presents as an intermittent fault nobody can reproduce.
A zone edit propagating to secondaries looks exactly like this and is fine within a few minutes. If the difference persists, a secondary has stopped transferring from the primary and is serving a stale copy of the zone to whoever happens to ask it.