What does Romans 9:12 mean?

This verse completes the long sentence begun in verse 10. Paul wants us to see that God's choice about which of Rebekah's twin boys would receive the covenant promises had nothing to do with their merits or their attributes. It was not about their works, since God declared His decision before they were born. It was not about their parents, since they came from the same two people.

And, now, Paul quotes what God said to Rebekah before the twins were born (Genesis 25:23): "The older will serve the younger." That means that God's choice to give the covenant promises to Jacob was also not about birth order. God simply chose to serve His own purposes before any human factors came into play.

Paul is building toward the argument that God decides who will receive His promises and who will not. His election, as mentioned in the prior verse, is something subject only to God's own sovereignty. Paul will use this concept to explain not only that God has the right to choose whom to save, but that He has the right and ability to save those who are not ethnically part of Israel: the Gentiles.