SSL Pinning is one of the most common iOS app security tips. But in order to understand what it means, you will first have to know how SSL works.
A browser attempts to connect with a website which is secured with a SSL. The browser then requests the web server to identify itself.
Web server then sends the browser its SSL certificate copy.
The browser checks if the SSL certificate must be trusted. If it can be, a message is sent to the web server.
Web server then sends back an acknowledgement to begin the SSL encrypted session.
The encrypted data is then finally shared between the browser and web server.