Peek and pop :
Another of the features within 3D Touch is called “Peek and Pop”, which allows for exactly what it suggests: peeking at selected notifications through a pop up screen without needing to open the specific app. Pressing and holding notifications offers a number of options depending on the which app the notification came from. For example, if a tweet, you’ll be able to retweet or like it, while an iMessage will allow you to read the message in a pop up screen and reply.
When inside apps, you can press lightly for a peek of the content, while continuing to press more firmly will go into the content.
In Mail, for example, you can press lightly to glance at a specific email then release the pressure to slip back to your entire list of emails. Pressing deeper when glancing at a specific email in the pop mode and you’ll be taken into the message where you will have access to all the actions you would have had if you had entered the Mail app and specific email manually.
From here, a light press on the event will either pull up your calendar, go to the incoming flight time if a flight number or allow you to peek at the web address within messages without going opening the browser.