You can think of light as a particle or a wave.
In the wave representation, it's a fluctuation in an electromagnetic field perpendicular to the direction of propagation. The amplitude of that fluctuation – the size of the maximum electric field at a particular point in the wave – varies according to an envelope that's kinda shaped like a bell curve.
In the particle representation, the photon's position is basically the point where the wave's amplitude is highest. Where the envelope is widest.