A keyboard LED is a status light on a keyboard, such as Caps Lock, Num Lock, or Scroll Lock. It tells the user the current state of a key or input mode, and in normal use it is only a convenience feature.
In cyber security, keyboard LEDs matter because a predictable light can become part of the attack surface. In restricted or air-gapped environments, an attacker with physical proximity or control over a peripheral may try to use the LED as a covert channel, sending or observing information through changes in light state. Defenders may also monitor or disable unnecessary indicators when reducing side channels. The key lesson is that even a small visible signal can matter if it can be influenced, observed, or repurposed to move information in ways the system was never meant to allow.