A light wave or an electromagnetic wave is a wave in the electromagnetic field. It does not require a physical medium to propagate through like mechanical waves (e.g., sound waves) do. Instead, electromagnetic waves consist of oscillating electric and magnetic fields that are self-sustaining and can propagate through a vacuum or empty space.
In the case of light waves, which are a subset of electromagnetic waves, the fluctuation of the electromagnetic field occurs in the form of oscillating electric and magnetic fields perpendicular to each other and to the direction of wave propagation. These oscillations occur in the transverse plane.
Unlike sound waves, which require a medium such as air, water, or solids to propagate, electromagnetic waves can travel through a vacuum, such as the vastness of space. This is because the electric and magnetic fields generate and sustain each other, creating a self-propagating wave.
In summary, the fluctuation of the medium for a light wave or an electromagnetic wave is the electromagnetic field itself. It is the interaction and oscillation of electric and magnetic fields that form the wave and allow it to propagate through space.