What does 'cursed be he that confirmeth not all the words of this law to do them' mean?
It means that anyone who fails to keep every last jot and tittle of the Law—which no mortal man can do—places himself under God's curse. The Law demands perfect obedience, and because humans cannot fulfill it, relying on the Law for favor brings the curse, not blessing.
In the supplied teaching, this verse is a warning: if a person says he will keep the Law in order to merit favor with God, no mortal man can do it, so he puts himself under the curse. Deuteronomy 27 says, 'cursed be he that confirmeth not all the words of this law to do them.' Les explains that if you do not keep every last jot and tittle of the Law—as no man can do, and only God can—you are under the curse of God. Paul makes the same reference in Galatians 3, saying that as many as are on the works of the Law are under the curse, because it is written, 'Cursed is every one that continueth not in all the things which are written in the book of the Law to do them.' The application is a tremendous warning: do not try to put yourself under the Law, since as a human you cannot keep it; putting yourself under a so-called legal system actually puts you under the curse of God. The Law commands, 'Do this, and do that,' but it cannot bring favor by human effort.
Cursed be he that confirmeth not all the words of this law to do them. And all the people shall say, Amen.