Force Touch is a touch screen technology that allows for increased functionality depending on how hard the user presses on the screen. For example, a harder press may bring up a different menu than a tap or tap and hold gesture. A Force Touch display has a layer of force sensors to detect pressure. The sensitivity can be adjusted to the point where it can even detect whether you are using a thumb or a finger, which can allow for even more functionality.
Force Touch was introduced with the Apple Watch and has been added to several devices since. The trackpads for both the 12-inch Retina MacBook and the 13-inch Retina MacBook Pro now incorporate Force Touch, and it is widely expected that the next generation of iPhones, the iPhone 6s and 6s Plus will both feature Force Touch displays.