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.

Les explains that in Acts 2:25 Peter goes right back into the Psalms. Peter says, “David speaketh concerning Him, that is, Christ,” and then quotes, “I foresaw the Lord always before my face, for He is on my right hand, that I should not be moved.” Les cautions that Peter is not talking about himself, nor is David; the quotation is about Christ. Earlier, Peter had charged his hearers with killing the Messiah, but God raised Him from the dead because death could not hold Him. Peter confirms Jesus’ death, burial, and resurrection and shows that God’s purpose was not thwarted; the promised King lives and can still return to give the Kingdom.

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.