Reading a certificate's fields
An SSL/TLS certificate's Subject identifies who it was issued to, the Issuer identifies the Certificate Authority that signed it, and the validity dates mark the window it's trustworthy in - browsers and clients reject a certificate outside that window, or one whose chain doesn't trace back to a trusted Issuer. This decodes a pasted PEM certificate directly, without needing to connect to the live server it came from, which is useful for checking a certificate file before actually installing it on a server.
What each field in the decoded output actually means
The Serial number is a unique identifier the issuing Certificate Authority assigned to this specific certificate. The Signature algorithm shows the cryptographic method used to sign it. The SHA-256 fingerprint is a short, unique hash of the entire certificate, often used to manually verify you have the exact certificate you expect rather than a substituted or mismatched one. Subject Alternative Names, when present, list every additional domain or hostname the certificate is also valid for beyond its primary subject.
Why checking the expiry date matters so much
An expired certificate causes browsers to show a hard security warning to visitors, often blocking access to the site entirely until it's renewed - which makes catching an upcoming expiration before it happens one of the most valuable, simple checks you can run on a certificate. This tool surfaces that status immediately and prominently at the top of the result.
Why decode a certificate locally instead of just checking the live site
Decoding a PEM file directly is useful before a certificate has even been deployed - checking a freshly issued certificate file from your Certificate Authority, verifying a certificate a colleague sent you, or confirming the contents of a backup certificate file, all without needing that certificate to already be live on a reachable server.