What does Acts 2:29 mean?

Peter is explaining that in Psalm 16:10, David is not speaking for himself but for his Lord (Acts 2:25–28). The speaker is sure God will not abandon Him to the land of the dead long enough for His body to decay. Peter points out that this can't be David. David died hundreds of years prior (1 Kings 2:10–11). Peter's audience even knows where his grave is.

David's tomb is in south Jerusalem, near Siloam. Nehemiah knew of it (Nehemiah 3:16). Josephus writes that in the second century BC, Hyrcanus I took three thousand talents from the tomb to pay Antiochus Sidetes to lift his siege of Jerusalem. A hundred years later, when Herod the Great heard what Hyrcanus had done, he opened the sepulcher to help himself to David's riches. Although he took some gold furnishings, he lost two men to a mysterious flame. In atonement, Herod built a monument of white stone at the door of the tomb. It's believed the tomb was destroyed in the Bar Kokhba revolt of AD 135. Some think it was rediscovered 1,000 years ago, and travelers to Jerusalem can visit the place now.

Peter identifies David as a "patriarch." David wasn't a patriarch of the Jewish people like Abraham, Isaac, Jacob, and Jacob's sons. But he did have a strong hand in establishing Israel as a nation, and he is the patriarch of the line of David of whom Jesus is the culmination and the promise.