Biomagnification
Biomagnification refers to the increased concentration of a toxic chemical the higher an animal is on the food chain. So for example....
Some toxic chemical is put into the ocean by accident such as pesticides or heavy metals makes their way into lakes.
The chemical infects a food source such as seaeweed.
A fish comes along and eats the seaweed, now the fish is infected. Also, the concentration is now higher in the fish then it was in the seaweed.
A bigger fish comes along and then eats the smaller one, causing the bigger fish to be infected with the chemical. Now, the fish's chemical concentration is higher in it.
As the chemical moves up the food chain, the trophic levels rise.
Now if a person were to eat the fish, they would get food poisoning and in worst cases die. This can be easily fixed though, by making sure our oceans do not get pesticides or any other chemicals.