In the context of light as a wave, the amplitude refers to the maximum displacement or intensity of the electric and magnetic fields comprising the light wave. It represents the strength or magnitude of the wave. In other words, the amplitude of a light wave determines the brightness or intensity of the light.
However, it's important to note that the concept of amplitude can be different depending on whether you are considering a classical or quantum mechanical description of light. In classical physics, light is described as an electromagnetic wave, and the amplitude refers to the maximum electric and magnetic field strengths. In this case, the intensity of the light is proportional to the square of the amplitude.
On the other hand, in quantum mechanics, light is described as a collection of particles called photons. In this quantum description, the amplitude of the light wave is related to the probability amplitude of detecting a photon at a particular location. The intensity or brightness of the light is related to the number of photons present, which is proportional to the square of the amplitude.
So, in summary, the amplitude of a light wave represents the maximum displacement or intensity of the electric and magnetic fields in the wave, determining the brightness or intensity of the light.