WHAT IS CERTIFICATE AUTHORITY?

Digital signatures rely on public and private keys. 
Those keys have to be protected in order to ensure safety and to avoid forgery or malicious use. 
When you send or sign a document, you need assurance that the documents and the keys are created securely and that they are using valid keys. 
CAs, a type of Trust Service Provider, are third-party organizations that have been widely accepted as reliable for ensuring key security and 
that can provide the necessary digital certificates.Both the entity sending the document and the recipient signing it must agree to use a given CA.