I understand the point of trying to find the density based on the isotropic point source, but to clarify does phi mean density and I means isotropic point source? To add on, what does the I necessarily represent at that point then?

@mylinhvu11 I think u r right. On this slide, the light source is obviously isotropic point source. I think the field can be represented as $\nabla \cdot F = const, \nabla \times F = 0$