The basic principle is that when you install an SSL certificate on your server and a browser connects to it, the presence of the SSL certificate triggers the SSL (or TLS) protocol, which will encrypt information sent between the server and the browser (or between servers); the details are obviously a little more complicated.
SSL operates directly on top of the transmission control protocol (TCP), effectively working as a safety blanket. It allows higher protocol layers to remain unchanged while still providing a secure connection. So underneath the SSL layer, the other protocol layers are able to function as normal.
If an SSL certificate is being used correctly, all an attacker will be able to see is which IP and port is connected and roughly how much data is being sent. They may be able to terminate the connection but both the server and user will be able to tell this has been done by a third party. However, they will not be able to intercept any information, which makes it essentially an ineffective step.
The hacker may be able to figure out which host name the user is connected to but, crucially, not the rest of the URL. As the connection is encrypted, the important information remains secure.
1. SSL starts to work after the TCP connection is established, initiating what is called an SSL handshake.
2. The server sends its certificate to the user along with a number of specifications (including which version of SSL/TLS and which encryption methods to use, etc.).
3. The user then checks the validity of the certificate, and selects the highest level of encryption that can be supported by both parties and starts a secure session using these methods. There are a good number of sets of methods available with various strengths - they are called cipher suites.
4. To guarantee the integrity and authenticity of all messages transferred, SSL and TLS protocols also include an authentication process using message authentication codes (MAC). All of this sounds lengthy and complicated but in reality it’s achieved almost instantaneously.