An SSL certificate, more accurately a TLS certificate in modern usage, helps establish encrypted communication between a browser and a server. It also helps the browser verify that it is talking to the expected site.
That is why certificate validity, expiry, issuer, and covered domain names matter during setup and troubleshooting.
What the Certificate Actually Does
The certificate is part of the trust chain used during the TLS handshake. It supports encrypted transport and identity verification at a domain level.
Why Expiry and Domain Coverage Matter
A certificate can still fail the real-world need if it expires, covers the wrong hostname, or has an incomplete trust chain. Those issues are common causes of browser warnings.
What a Browser-Side Checker Can and Cannot Do
Educational certificate tools are useful for understanding expiry math and field meanings, but real certificate inspection still depends on actual browser or server-level checks.