iMore has a great summation of all the new stuff you’re free to with 3D Touch.
you’re free to use the feature from the home screen, in apps, and more.
Peek and Pop:Peek and Pop allows you to preview certain things in iOS.
Its basically Quick Look for iOS.
App Switching:App switching gets a little faster with 3D Touch.
Instead of double-tapping the Home button, you could now use 3D Touch.
Trackpad:you might now get a little trackpad to make selecting text easier.
Just press firmly on the keyboard, then swipe around to move the cursor.
If you press a little harder you might select also text.
Thats just the basics and Im sure well see a few hidden features pop up over the next week.
How to use 3D Touch on iPhone 6s| iMore