A photon is a single quantum of light. In practical terms, it is the tiny unit that carries energy through an optical channel, whether that channel is a fiber, a laser beam, or another light-based link. In networking, photons are the physical carriers behind optical transmission, even though engineers usually talk about wavelengths, modulation, and signal power instead of counting individual particles.
Photons matter in cyber security because many secure and high-speed systems depend on light-based communication. Defenders use optical links to move data with low electrical interference and, in some environments, to reduce tampering risks. Attackers can also exploit optical paths: laser-based eavesdropping, device fault injection, and side-channel analysis all rely on the fact that light can be measured, reflected, or disrupted. When a system uses photons to move information, protecting the optical path is part of protecting the data itself.