It means that, in addition to the regular finger tap we've all been using on our smartphones and the harder Force Touch press that's featured in the Apple Watch and MacBook, users will be able to use a "deeper press" on the iPhone 6S.
Gurman notes that app developers might use three levels of touch input for games.
For the 6S, Gurman has suggested that Force Touch could be used to offer useful shortcuts. For example, if you use Force Touch on a location in Apple Maps, your iPhone will automatically start navigating you to that location instead of making you go through the three extra steps that are usually needed to start navigation in Apple Maps.
Some are calling the iPhone 6S' version of Force Touch "3D Force Touch."
Force Touch has many useful applications on the Apple Watch. For example, instead of crowding the Apple Watch's small screen with menu and options buttons, you can use Force Touch to bring up those options with a hard press, thus leaving more space for useful information on the screen.
And on a MacBook, one of the many uses for Force Touch include showing you a preview of a website when you press hard on the link.