They forbid any new states that were formed from choosing not to allow slavery. (Article IV, Sec 3.3)

​

(3) The Confederate States may acquire new territory; and Congress shall have power to legislate and provide governments for the inhabitants of all territory belonging to the Confederate States, lying without the limits of the several Sates; and may permit them, at such times, and in such manner as it may by law provide, to form States to be admitted into the Confederacy. In all such territory the institution of negro slavery, as it now exists in the Confederate States, shall be recognized and protected be Congress and by the Territorial government; and the inhabitants of the several Confederate States and Territories shall have the right to take to such Territory any slaves lawfully held by them in any of the States or Territories of the Confederate States.