Anything that's polluted is ruined and dirty — it's been contaminated by something dangerous or even deadly. It's not safe to eat fish caught in a polluted river.
The adjective polluted is obviously a close relative to pollute and pollution, so you know that polluted air or water isn't clean. All of these words come from the Latin polluere, "to soil or defile," from a combination of por-, "before," and -luere, "to smear." A polluted city can result from a coal-burning factory, and things can also be figuratively polluted: "Fear of outsiders created a polluted society."