The subject field identifies the entity associated with the public key stored in the subject public key field.
rfc-editor.org/rfc/rfc5280#section-4.1.2.6
In Transport Layer Security (TLS) a certificate's subject is typically a computer or other device, though TLS certificates may identify organizations or individuals in addition to their core role in identifying devices.
en.wikipedia.org/wiki/Public_key_certificate
The subject, in security, is the thing being secured.
In this case it could be a person's email or a website or a machine.