It is a third-party organization responsible for managing and issuing certificates, just like the intermediary company C in the example. Generally speaking, CA must be trusted and recognized by all industries and all the public. Therefore, it must have enough authority. Just like Company A and Company B have to trust Company C to find Company C as the intermediary of the official seal.
A digital certificate is actually a record stored in a computer, and it is a statement issued by CA, which proves the unique correspondence between the certificate subject ("certificate applicant" becomes "certificate subject" after being issued with a certificate) and the public key contained in the certificate. The certificate includes the name and relevant information of the certificate applicant, the applicant's public key, the digital signature of the CA that issued the certificate and the validity period of the certificate. The function of digital certificate is to make the two sides of online transaction verify each other's identities and ensure the normal conduct of e-commerce.