This feature had been rumored to appear in the Galaxy Note 8 and iPhone X, but didn't.
When your phone is locked, you press your finger on the part of the screen where a physical button would be.
It lights up to capture an image of your print, then unlocks the phone and gets out of the way.
This technology lets the full display take up almost all of the phone face, without making you reach for a sensor on the back.
The tech, made by Synaptics, technically lives beneath the display.