What does photon noise mean in digital image processing or computer vision?
Ultimately at a fundamental level, an image signal as captured by a digital sensor is obtained by integrating the energy transmitted by the photons of the light wave. 

If you are imaging a bright scene, there will be a large enough number of photons hitting the detector that this effect will not be detectable, but if the sensor is operating in a dark environment, individual photons may be detected and counted, and the number of photons that have hit a cell of the detector as a function of time is modeled as a Poisson random process.