If you lock your device, turn it off or exit the camera, your device will no longer perform as a Wi-Fi video camera. If you wish to make the screen go dark, tap Dim Screen at the bottom of the display. The camera will still be running, but the screen will appear dark. Tapping the screen once will re-enable it. Since the device is always on, be sure to keep it plugged into power. Also consider enabling Do Not Disturb mode in your settings.