Polygon density refers to the relative size of polygons in a particular area of a mesh. Areas of high detail usually require a higher polygon density than areas of low detail. Examine the polygon density of the character model in the following screenshot:
High polygon density is used for areas of high detail (eyes and hands), while low polygon density is used for areas of low detail (snout and belly)
As we can see, the character has a higher polygon density near the eyes, but a lower density on his belly.
Within any given area, polygons should all be approximately the same size as one another so as to avoid shading issues
