"Fellow Israelites, I may say to you confidently of our ancestor David that he both died and was buried, and his tomb is with us to this day.  Since he was a prophet, he knew that God had sworn with an oath to him that He would put one of his descendants on his throne.  Foreseeing this, David spoke of the resurrection of the Messiah saying, 'He was not abandoned to Hades, nor did His flesh experience corruption.'  This Jesus God raised up, and of that all of us are witnesses."

Peter points out the fact that king David had a throne and is still in it.  He does this as a means of saying:  Our Savior, Jesus, is not in the throne anymore.

He furthers his point by letting the crowd know about David's prophetic word:  The Savior's body would not be corrupted.  Jesus would not be abandoned in Hades. 

Jesus would be raised up by God.

All of the salvation message relies on this point.  Jesus, God, is stronger than death.

To understand this point, you must go back to the garden of Eden.  When man sinned, we were condemned to death.  Adam was, you are, I am...

Jesus conquered all of that death.  He has no grave.  He is the Saving One!