In assymetric encryption, you have a private key with which you encrypt messages and your receivers have public keys with which they can decrypt them. The receiver wants to ensure that the sender is the one who he claims to be. To build in trust between the sender and receiver, the sender can ask for a certificate to a Certification Authority (CA), after providing this organization with documents that proof his identity. The CA issues to the sender a certificate, containing his public key some identification information and a digital signature created using the private key of the CA. The sender can then use this certificate in communicating messages. The receiver uses the public key of the CA to check on the validity of the public key of the sender. The whole idea is that if the receiver trusts the CA, he trusts all the certificates the CA issues.