Mutual authentication refers to two parties authenticating each other at the same time. That is a client authenticating itself to a server and that server authenticating itself to the client in such a way that both parties are assured of the others' identity. In adition to SSL, muutual authentication provides authentication and non-repudiation of the client, using using digital signatures. This process it performed with certificates interchange. That is both client and server send its own certificates in connection handshaking, the client validate if the server certificate is valid and then the server validates the client certificate validation. If all it's ok the connection is stablished. After this, the server can read client centificate information to perform client identification.