It just means that it was written very simply so that the people of that time could understand it.
And the people of THIS time??
NIV Romans 15:20-21
20. It has always been my ambition to preach the gospel where Christ was not known, so that I would not be building on someone else's foundation.
21. Rather, as it is written: "Those who were not told about him will see, and those who have not heard will understand."
NIV 1 Corinthians 2:12
12. We have not received the spirit of the world but the Spirit who is from God, that we may understand what God has freely given us.
NIV 2 Corinthians 1:13-14
13. For we do not write you anything you cannot read or understand. And I hope that,
14. as you have understood us in part, you will come to understand fully that you can boast of us just as we will boast of you in the day of the Lord Jesus.