Enter a domain to check the public registry database: registered domains show their registrar and expiry; unregistered ones report back as not found.
The checker queries RDAP, the Registration Data Access Protocol, the modern replacement for WHOIS, which is the authoritative public record registries publish for each domain. If the registry returns a record, the domain is registered, and the record usually includes the registrar, creation date and expiry date. If the registry answers 'not found', the domain appears available to register through any registrar, wording that is deliberately careful, because premium, reserved and registry-held names can also return no public record while not being freely purchasable.
RDAP coverage is broad for the major extensions (.com, .net, .org, most country codes) but not universal; a few registries do not publish RDAP at all, and for those the checker reports that it could not determine the status rather than guessing. Nothing about the search is recorded here, the query goes from the browser to the public RDAP service.
It says 'appears available', can I definitely register it? Very likely, if it is an ordinary name in a covered extension. Registrars make the final determination at checkout, where premium pricing or reservations show up.
The domain is registered but the site is empty, can I get it? A parked or unused domain is still owned until it expires and is released (or the owner sells). The expiry date shown is the earliest point release could happen, and popular names are usually renewed or auctioned.
Which extensions are supported? Any extension whose registry publishes RDAP, most major ones. If the lookup reports it could not determine the status, that extension's registry is one of the remaining holdouts.