The Christian is to avoid the pollutions from the world (2 Pet 2:20). The Jerusalem council warned against the pollution of idols, namely eating food that first had been offered to an idol, lest this offend others who were weaker (Paul later explained this in detail in Romans 14); also included is fornication, which is always wrong, and eating strangled animals and blood as forbidden in the OT.
International Standard Bible Encyclopedia (1915)
In Mal 1:7, "Ye offer polluted bread," i.e. not actually unclean, but worthless, common (compare Ezr 2:62), bread here being used metonymically for sacrificial offerings generally (compare Le 21:6; Mt 6:11). The phrase in Ac 15:20, "the pollutions of idols," is explained in 15:29 by "things sacrificed (the King James Version "meats offered") to idols."