Jamieson, Fausset and Brown on Acts 15:20

20. But ... that they abstain from pollutions of idols--that is, things polluted by having been offered in sacrifice to idols. The heathen were accustomed to give away or sell portions of such animals. From such food James would enjoin the Gentile converts to abstain, lest it should seem to the Jews that they were not entirely weaned from idolatry.

and from fornication--The characteristic sin of heathendom, unblushingly practiced by all ranks and classes, and the indulgence of which on the part of the Gentile converts would to Jews, whose Scriptures branded it as an abomination of the heathen, proclaim them to be yet joined to their old idols.

and from things strangled--which had the blood in them.

and from blood--in every form, as peremptorily forbidden to the Jews, and the eating of which, therefore, on the part of the Gentile converts, would shock their prejudices. See on Ac 15:28.