Drawing what your app would look like on a bunch of paper and asking people to pretend it's a real app. This saves you the time of making something on your computer...(which ironically would actually take less time for alot of people).
My low fidelity prototype flew away when the wind got really strong when I was testing my app for hobos at Golden Gate Park.
RobotCowboy
I would actually call this "Paper Prototype". Low-Fidelity can be digital.