POP stands for "persistent organic pollutant".
POPs are supposedly toxic chemicals that adversely affect human health and/or the environment around the world. They can persist or accumulate for long periods of time in the environment. In a 2001 treaty known as the Stockholm Convention, participating countries agreed to reduce or eliminate the production, use, and/or release of 12 key POPs, the full list of which can be found on the EPA website.
One controversial example of a chemical placed on the POP banned list is DDT, which has been responsible for saving millions of lives around the world by helping to eliminate malaria in regions where it has been used.