Arsenic
Arsenic is a toxic element found in soil that can harm plants, animals, and people.
Think of it this way
Arsenic in soil is like broken glass mixed into a bag of flour. The flour looks fine, but glass hides inside every cup you scoop out. Plants pull arsenic up through their roots the same way. It gets into what they grow, even when you cannot see it.
Explaining arsenic by grade level
Some dirt has bad stuff in it that you can not see. Old farms used sprays that left this bad stuff behind. If you test the soil, you can find out how much is there. Plants grown in that dirt can take it up through their roots.