(Act 10:9) On the morrow, as they went on their journey, and drew nigh unto the city, Peter went up upon the housetop to pray about the sixth hour. Act 10:10) And he became very hungry, and would have eaten: but while they made ready, he fell into a trance,(Act 10:11) And saw heaven opened, and a certain vessel descending unto him, as it had been a great sheet knit at the four corners, and let down to the earth. Act 10:12) Wherein were all manner of fourfooted beasts of the earth, and wild beasts, and creeping things, and fowls of the air.(Act 10:13) And there came a voice to him, Rise, Peter; kill, and eat.
God tells Peter to kill and eat all manner of things.
(Act 10:14) But Peter said, Not so, Lord; for I have never eaten any thing that is common or unclean.
Peter wasn't sure that he should eat.
(Act 10:15) And the voice spake unto him again the second time, What God hath cleansed, that call not thou common.
(Act 10:16) This was done thrice: and the vessel was received up again into heaven. (Scriptures from KJV.)
Peter is told effectively that God has made all things clean. "Peter naturally referred to the well known and admitted written Law. One design of the vision was to show him that that Law was now to pass away." (Barnes)
Peter was being shown that the old law was ended and now there was no division between Jew and Gentile.