SSL certificates are developed based on public-key cryptography, which encrypts information with digital keys so that the information can only be read by intended recipients after decryption.
A key pair consists of a public key and a private key. The public key may be publicly distributed by a user, while the private key is kept by the user. Information encrypted with the public key can be decrypted only with the corresponding private key, and vice versa.
An SSL certificate actually represents the verification of the public key from CA, which contains digital signing authority information, user information of the public key, the public key, authority signature, and expiration date.