Acts 3:21 says, “whom heaven must receive until the period of restoration of all things about which God spoke by the mouth of His holy prophets from ancient time.”

This is most probably a reference to the future period when God will restore the earth to its original condition which existed before the fall of Adam and Eve.  We see this reflected in Paul’s comments as well.

Rom. 8:20-21, “For the creation was subjected to futility, not of its own will, but because of Him who subjected it, in hope 21 that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God.”

Another possibility, though less likely, is that it refers to the restoration of the Kingdom of Israel mentioned by Paul in Rom. 11:25-26.

Rom. 11:25-26, “For I would not, brethren, have you ignorant of this mystery, lest ye be wise in your own conceits, that a hardening in part hath befallen Israel, until the fullness of the Gentiles has come in; 26 and so all Israel shall be saved: even as it is written, There shall come out of Zion the Deliverer; He shall turn away ungodliness from Jacob.”

Still, the first option seems most probable and is further supported by 1 Cor. 15:25-26 which says, “For He must reign until He has put all His enemies under His feet. 26 The last enemy that will be abolished is death.”