If you are an old jailbreaker, I`m pretty sure you remember Zephyr. With 3D Touch, Apple just brought that back and we are loving it!
With the new feature called 3D Touch, Apple is basically bringing to iPhones the Force Touch technology.
Using 3D Touch, you are able to access shortcuts for your apps and also have preview of emails, messages and so on. Think about it as a more advanced right click for the iPhone!