Face Unlock, a security feature introduced with Ice Cream Sandwich, while a fun concept, proved vulnerable to trickery. Specifically, the unlock method would recognize photos of your face as if they were your real face. Another issue with Face Unlock was that it ostensibly never locked users out after numerous failed attempts.
Looking to address these issues, Google did a bit of tweaking to Jelly Bean's Face Unlock. Namely, FU now features a "Liveness check" option which, as the name suggests, makes sure you're a real, live person before unlocking your device.