Q: What does SSL mean in iPhone 6 settings?

SSL means Secure Socket Layer, it is a secure transmission protocol for data.  The email provider has to tell you if they use SSL or not, and you have to turn on or off SSL accordingly.  Ask your email provider how they want you to set that on your iPhone.

SSL - 'Secure Sockets Layer' - is a protocol for email encryption. It has to set to on for iCloud email to work, though when you sign in in Settings>iCloud and enable email it should set it all up automatically.

If you are setting up an account on Verizon you want SSL turned ON.

If you do NOT have SSL enabled on ALL of your email accounts you should never use a public WiFi such as Starbuck's, because you will almost certainly have your password stolen. You should only use WiFi hotspots that support encryption.