He is speaking about the incident which occurred in Acts 10:9-17. Peter, in response to the vision which bade him to kill and eat animals considered "unclean” by the Law of Moses, protests that he has never violated the dietary restrictions under the Law. The Angel responds that he must never continue to call "unclean” what God has declared to be "clean.”
This was to prepare him for the preaching of the Gospel to the gentiles whome the Jews considered to be "unclean” because of their idolatry. Shortly after that, Peter was sent to convert the gentile Cornelius and all his household. Cornelius, an officer in the Roman Legion, and perhaps many of his household - family and servants - were gentiles but gentiles who revered the God of Israel.
I hope you have found this helpful.