What is a Certificate Authority?

"In cryptography, a certificate authority or certification authority (CA) is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate. This allows others (relying parties) to rely upon signatures or assertions made by the private key that corresponds to the public key that is certified. In this model of trust relationships, a CA is a trusted third party that is trusted by both the subject (owner) of the certificate and the party relying upon the certificate."

So Certificate Authority is a entity we all trust. This allows to check and verify other user certificate and do lots of maintenance of whole PKI infrastructure.

That trust allows Certificate Authority to play a intermediator role in data exchanges. It allows to verify signatures, to trust certificates, to build secure information infrastrusture. PKI rollout begins with setup of Certificate Authority. You can choose to use publicly available Certificate Authority or create your own.