It helps to have some history of the book of Galatians. Some Jews had accepted Jesus as their Messiah, but were not willing to accept that the Law given by God had been replaced. They insisted that it was necessary for salvation that Christians keep the Law.
The Apostle Paul heard of this and wanted to set this important matter straight. He wanted them to know that "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!” (Acts 4:12)
Then Paul said in Galatians 3:13, "Christ redeemed us from the curse of the law, having become a curse for us. For it is written, 'Cursed is everyone who hangs on a tree,’” (Quoted from Deuteronomy 21:23.)
The curse that Paul is speaking of is the curse of the Law, more specifically the curse for not keeping the Law.
Paul is showing that by taking the severest penalty for breaking the Law, dying hanging on a tree, Jesus took the penalty for everyone who broke the Law (and this could only be broken by those who accepted this Covenant from God, as the Jews had said "all that he Lord has commanded we will do" – Exodus 19:8).
This curse was added on to the original curse given to Adam, when he disobeyed God by eating the fruit he was told not to. Adam was told if he disobeyed he would die.
Jesus took the full penalty for both curses. He died to take the place of Adam to redeem all of Adam's race, and he did this on a tree to redeem the Jews from the curse of the Law. This made Jesus a Ransom for All…both Jews and Gentiles (1 Timothy 2:6).