In Acts 2:25, whom is David speaking about in the Psalm quotation?

In Acts 2:25, David is speaking concerning Christ, the Messiah. Peter explains that David's Psalm quotation refers to Christ, not to David himself, as David foresaw the Lord always before his face, rejoicing in hope. This confirms Christ as the promised Messiah raised from the dead.

Les teaches that in Acts 2:25, David goes right back into the Psalms and speaks concerning Christ. Peter quotes David's words—'I foresaw the Lord always before my face, for He is on my right hand, that I should not be moved'—to show the quotation refers to the Messiah, not to David himself. Peter is not talking about himself, nor is David. This follows Peter's earlier point that though they killed their King and Messiah, God raised Him from the dead, because it was not possible that He should be held by death. The context emphasizes that God raised Him and He is alive, still able to give the Kingdom.

Acts 2:25

For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:

When Peter quotes Psalm 16 in Acts 2:25, is David speaking about himself or about Christ?

David is speaking about Christ, not himself. In Acts 2:25 Peter quotes Psalm 16 and identifies David's words as concerning Him—that is, Christ. The context stresses that Peter is not speaking about himself, nor is David; the Psalm points to Christ, not David.