The prophets knew a great deal; more than many give them credit for. They knew that God was going to send the Messiah, the Christ, into the world and that he would be a suffering Saviour, fulfilling all the Old Testament types and making an atonement for his people.
The Bible is not like any other book, because God is not like any other author. It contains revelation of the future which only God knows and only he can reveal. If then we treat the Bible as if it was written by uninspired human beings whose understanding was limited, and if we say they could not have spoken better than they knew, we no longer have an inspired Bible in our hands. This verse tells us the prophets came to be instructed by their own prophecies since they knew them to words which God had spoken through them and not words that came only from their own understanding.
Never forget that Christ was the author of the Old Testament. That's why, when we read the Old Testament and we read a prophet or we read the books of Moses, we don't say – as we do it when we are analysing human literature – ‘I must find out about the author of this book of the Bible. I must find out about Isaiah or Jeremiah, because, like human authors, they will have been influenced by their childhood, and what they learned when they grew up, or what historic events they witnessed, and this will colour their writings and help us to understand what they mean.’ We don't analyse divine literature like that in a humanistic way, as we would if we were studying Shakespeare or anyone else in time, because Isaiah wasn't the author, ultimately, and Jeremiah wasn't the author. The Spirit of Christ which was in them spoke; Christ was the divine author of it all. So we don't stoop to human technicalities in the interpretation of the Bible. Many try to and it leads them astray. This is like no other book.