SSL uses a method called public-key encryption. In a public-key encryption two keys are used. One for encrypting data (public key) and one for decrypting data (private key). A server sends its public key to any requesting client. This key is used by the client to encrypt data sent to the server. When the server receives the encrypted data from the client (called ciphertext), the server uses its private key to decrypt the ciphertext.
Conversely, a client can also send a public key to a server so that the server will encrypt data sent to the client. When the client receives the encrypted data from the server, the client uses its private key to decrypt the ciphertext.