The previous video dealt with Gauss's law and conductors, where, when in electrostatic equilibrium, the electric field = 0 inside. Any net charge sits on the outer surface of a metal chunk. But, when we move to NON-conducting materials, things are more involved since there are NO de-localized electrons. It is OK to have net charge inside the material, spread throughout the volume, and therefore it is OK to have an electric field inside these materials. There would still just be static charge, and no currents. This is also the case where we would have charge densities. Check out how to do this for uniform charge densities, and I hope it helps while you review this topic.