What this checks
It loads the site over HTTPS (which only succeeds with a valid, trusted certificate), checks that http redirects to https, and reads the domain's current certificate from public Certificate Transparency logs, where every trusted certificate is recorded.
Most certificates today last 90 days or less and renew automatically. An expiry under two weeks away usually means automatic renewal is failing.
Questions
Where does the certificate data come from?
Certificate Transparency logs, via SSLMate's public Cert Spotter API.
Does it test TLS versions and ciphers?
No. For a deep TLS audit, use a dedicated scanner like SSL Labs.
Why does it show several certificates?
Sites often have overlapping certificates during renewal, or separate ones per CDN. The newest valid one is shown first.
Does HTTPS affect SEO?
Google uses HTTPS as a light ranking signal, and browsers warn on sites without it.