This means that Peter is about to explain the meaning of what the people had witnessed. This can be stated in active form. Alternate translation: "know this" or "let me explain this to you" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Peter was referring to what he was saying. Alternate translation: "listen carefully to what I am saying" (See: [[rc://en/ta/man/translate/figs-metonymy]])