The tiny little led-light mostly located at the top of the front on Android phones featuring them. Notifications can use them to flash/blink/burn in different colors.
What is a notification-led?
This little LED light which, if your device features one, in most cases is in the upper right corner of the device's front, can be used in multiple ways. Intended for notifications, it can flash/blink/burn in different colors, such indicating different kind of notifications -- e.g. blinking green for a missed call, magenta for an arrived message, red for a battery-low warning, etc. Some devices/apps allow the user to configure this, some apps even can use it for their own purposes.
What to use this tag for?
If your issue is with the above described notification LED, of course.
What not to use it for?
Though there is the synonym "led" attached to it, the LED flashlight is something completely different. For this and the "torch", rather use the camera-flash tag.