How do I use the Peek quick actions of 3D Touch?

The "Peek" feature in 3D touch allows you to preview websites, email and even photos without having to leave an app or without having to open links. You can also use the peek feature to perform additional tasks such as replying or forwarding an email by invoking "Peek quick actions." To do so lightly press the link or item you want to preview, then swipe up to open the pop up box of additional deep links and features.