In Android 4.0, Google introduced facial recognition, which could be used to unlock your device. Android 4.1 Jelly Bean improves on the formula, with a "Liveness check" feature to make sure someone's not trying to break in with your picture. Here's how to set it up.
In Settings, choose Screen lock. Tap the option for Face Unlock. A screen will pop up with a few reminders, namely that Face Unlock is less secure than the other methods for unlocking your device (although it is a lot faster and more convenient). Click Set It Up, and Continue.
The screen will show a live camera view, with an outline where your face should be in the frame. For best results, hold your Nexus 7 at about arm's length, and make sure the light is coming toward your face, not from behind your head. Once you're centered in the outline, stare directly into the camera lens rather than at the Nexus 7's screen. Hold still for a few seconds, then you'll get a confirmation that your face has been captured.
You'll then be asked to choose a backup unlocking method, in case Face Capture doesn't work. Set a PIN or use the Pattern option to create a custom unlock gesture.
For added security, activate the Liveness check in Settings > Security. It requires a blink when verifying your face, which will keep Face Unlock from being tricked by a photograph of you, a widely publicized hack that was discovered when Face Unlock debuted in Android 4.0