Visibility
Visibility is how far and how clearly you can see objects through the air around you.
Think of it this way
A glass of clean water is clear all the way through. Add a pinch of flour and stir. The flour clouds the water, and you can't see the other side. More stuff in the air or water means less you can see.
Explaining visibility by grade level
Think about car lights on a foggy night. When fog is thin, you can see the red lights from far away. When fog is thick, those same lights look blurry and close. How much fog is in the air changes how well you see.