It is a toxic industrial chemical, it was excreted in industrial wastewater into streams & sewer systems in America - & other places I imagine - & contamined groundwater & streams & lakes & continued on to the oceans.
People drank the contaminated wellwater & swam in these lakes & ate the fish they caught in these streams.
The chemical is retained in the fatty tissues in fish and in the animals that eat fish like eagles & otters, etc..
PCBs are also absorbed into the silt & mud on the bottom of rivers & lakes.
It takes decades if not longer to dissolve or break down so it must be dredged out. It is somewhat similar to dioxin.
The United States Environmental Protection Agnency has a fact sheet on the subject (in English).
I believe that if you Google "PCB" - all capital letters - & the word "chemical" in most languages Google should be able to return some relevant sites. You might also add the word "Monsanto" - this corporation was the major source of PCB pollution in America.