How the availability check actually works
This queries the domain's authoritative WHOIS server directly and checks whether it returns an active registration record or a clean "no match" response - the same fundamental signal registrars themselves rely on to determine availability. It's a fast, reliable first check, not a guarantee: the only way to be completely certain a domain is purchasable right now is to attempt registering it through an actual registrar, since availability can change in the time between any check and an attempted purchase.
Reading the result
A green "Likely available" result means no active registration record was found in WHOIS for that domain. A red "Already registered" result means an active record exists, and where possible, the tool also shows which registrar the domain is registered through.
Why "likely" and not "guaranteed"
Two things can create a gap between a WHOIS check and reality: domains can be registered by someone else in the brief window between your check and an actual purchase attempt (domain squatters specifically watch for this with valuable names), and a small number of registries delay publishing brand-new registrations into WHOIS. A registrar's own live registration system is always the final, authoritative answer.
Coverage across different extensions
Common top-level domains (.com, .net, .org, and similar) are queried directly against their known WHOIS servers. For less common extensions, the tool falls back to asking IANA which WHOIS server is authoritative for that TLD. A small number of country-code TLDs restrict public WHOIS access entirely for privacy reasons, in which case the lookup will fail regardless of the domain's actual availability.