A layer of fog is typically no more than around 600ft thick, so when a pilot flying above fog looks downwards they can usually still see the ground features below. However, when the time comes to begin a descent or when on final to land, the pilot is now looking across the ground at a much shallower angle and through significantly more of the fog!
The visibility at an angle like this is known as Slant Visibility and it has been a factor in many aviation accidents.
A pilot who arrives overhead an airfield may assess the visibility to be good, as they can see the runway below them. However, once on final approach the slant visibility looking towards the runway is significantly worse. The pilot may continue the approach to a dangerously low height in poor visibility as they are expecting the visibility to improve closer to the airfield!