See the records public DNS publishes for a domain: nameservers, addresses, mail, and authentication.
A DNS check reads the live records for a name. Nameservers say who hosts the zone. A and AAAA say where the site points. MX says where mail is delivered. SPF, DMARC, and DKIM say which servers may send mail for the name.
Use it before you move hosting or email. If the nameservers are not yours, changing records in the wrong panel does nothing. If MX is empty, mail to the domain has nowhere to go.
Nameservers, IPv4 and IPv6, the www record, MX, SPF, DMARC, other TXT records, and DKIM on common selectors.
It is not published, or it was added too recently and has not reached public DNS yet. Wait for the TTL and check again.
A leading www is removed. The lookup is for the domain itself, plus a separate www row.