Explanation: The small white lamp next to the shutter release button lights up if the ambient light is possibly too low for the autofocus and:
- you have not deactivated it in the custom setting menu a2 Built-in AF-Assist illuminator and
- the focus point selection is either automatic or manually with the center focus point selected
- the autofocus is not switched to continuous focus tracking.
Additionally it will light up if the flash is set for red-eye reduction and it will blink while the self-timer is counting down.
Recommendation: I usually leave it off as the light may be disrupting. Taking photos without the light is more discrete and the autofocus works still fine in many situations where the camera would switch on the additional light.