POPs are persistent organic pollutants.
They include various chemicals. They are so nasty that many, like DDT, have already been banned in some countries. One of the problems is that they are lipophilic, which means that they accumulate in the fatty tissue of living animals and human beings. They also magnify, which means that their concentration increases in animals at the top of the food chain. When they enter the body they don't leave it, they are stored in fat.