[chrisdroukas]

A long press is a function of time: "How long has the user pressed?"

3D Touch is a function of pressure: "How hard has the user pressed?"

In practice, things that your application previously accomplished with long presses may be more suitably accomplished with 3D Touch — things like contextual menus or shortcuts come to mind. Additionally, iOS has new APIs for presenting content using 3D Touch. The article briefly references them.