Learn in the textbookThe Certificate Authority and the Operation and Revocation of Certificates
DefinitionA CA (Certificate Authority) is a body that issues digital certificates binding a public key to its owner, and vouches for their contents with its own signature.
ExplanationA public key on its own tells the recipient nothing about whether it really belongs to the person who claims it. So the CA first checks the applicant's identity, then creates a digital signature over the public key and owner details with its own private key and hands them over as a digital certificate. By verifying the CA's signature, the recipient can confirm that the key's owner is who they claim to be.
AnalogyIt is like registering an official seal at a government office: the office checks who you are and then certifies that this registered mark (the public key) really is yours. A neutral third party (the CA) does the same for keys.
Memory hookC is for Certificate, A is for Authority. Read it as "the authority over certificates" and it is clear this is the side that hands them out.