The Bible says "you were born in sin". What sin?

Sins is disobeying a law of God. God said Adam and Eve could eat of every fruit from every tree of the Garden of Eden except the tree of knowledge of Good and of Evil. With a challenge for more wisdom and knowledge by the devil they chose to eat of the fruit. The moment they did the physical act of ate the fruit they broke God's law. From that one act sin entered into all the world by default since all of their children would be born in sin. Sin separates man from God. Thousands of years Jesus Christ undid what Adam and Eve did so that man can once again have a righteous and holy relationship with God.