Permeability
Permeability is how easily water or other liquids can pass through a material like soil or rock.
Think of it this way
A kitchen strainer lets water pass through its many small holes quickly. A solid bowl blocks water and holds it in place. Soil works the same way — loose, open soil lets water drain fast, while packed clay soil blocks it like a bowl. The more open paths a material has, the higher its permeability.
Explaining permeability by grade level
Some rocks have tiny holes you cannot see. When you pour water on them, the water soaks right in. Other rocks have no holes, so water sits on top. That is why dirt soaks up rain but a smooth stone does not.