This passage teaches that when you read Scripture, what you are reading does not merely come from a man but also from God. The Bible is the writing of many different men. But it is also far more than that. Yes, men spoke. They spoke with their own language and style. But Peter mentions two other dimensions of their speaking.
First, they spoke from God. What they have to say is not merely from their own limited perspective. They are not the origin of the truth they speak; they are the channel. The truth is God’s truth. Their meaning is God’s meaning.
Second, not only is what they spoke from God, but how they spoke it is controlled by the Holy Spirit. Men, moved by the Holy Spirit, spoke from God. God did not simply reveal truth to the writers of Scripture and then depart in hopes that they might communicate it accurately. Peter says that in the very communicating of it they were carried by the Holy Spirit. The making of the Bible was not left to merely human skills of communication; the Holy Spirit himself carried the process to completion.