Subject

What it means

Who or what this certificate belongs to — a hostname for TLS certificates (in the Common Name and, authoritatively, the Subject Alternative Names), or a person/organization for signing certificates.

Why it matters

If the subject doesn't describe the thing you're securing, the certificate is the wrong one. Browsers match the hostname you visit against the certificate's names — a mismatch is the classic NET::ERR_CERT_COMMON_NAME_INVALID error.