Peek and Pop are two functions available on newer iPhones (iPhone 6s and above) that allow you to preview web pages, messages, and more without interrupting what you’re doing, providing the chance to decide whether to commit to opening them fully. For example, if somebody sends you a web link or address in Messages, you can get a sneak peak of the content without leaving the app for Safari or Maps.
Taking advantage of the 3D Touch pressure sensors, press firmly on a link or item in a list to peek at the contents. This will bring up an overlay showing a preview of the content. You can then either let go to dismiss the preview or press the screen even harder to pop it full screen. You can also swipe up from the preview for additional options like Reply and Notify Me. Some apps allow you to swipe left or right for other quick actions, such as trashing an email from Mail.
It can take a few tries to get used to the force needed to activate the peek and pop functions, but with a bit of practice it’s a very useful 3D Touch gesture to have mastered!