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.
The wave has two properties we care about. The first is the propagation velocity: the rate at which the wavefront moves forward. The wavefront is the furthest-forward point at which the wave has effects.
The second property is the group velocity, which is the speed at which the envelope moves forward.
In a vacuum, these are both equal to c (3e8 m/s).
Now, say you have a soup of polar molecules bouncing around randomly. Each molecule has a negative and a positive end. They have no common orientation, because there's no field to orient them.
If there were a standing (stationary) electromagnetic wave in this medium, it would cause particles to line up in its direction where it was strongly positive, and line up opposite to its direction where it was strongly negative.
Light isn't a standing wave, though. It moves forward.
As it moves forward, the regions of strongest positive and negative strength move with it.
This means the dipoles in the soup are constantly being aligned and realigned in opposite directions, very rapidly.
An accelerating charged particle releases radiation, in the form of more EM waves.
That radiation produced by the accelerated dipoles coheres in a wave with the same propagation vector but a slightly different phase than the original wave. When this new radiation and the original wave interfere, you get a wave with a slower group velocity than the original wave had.
Remember that when we switch back to the particle representation, we say that the particle is located at the point of highest amplitude. So the photon moves forward with the group velocity, which is slower than c.
