What does the phrase the union is dissolved mean

The election of Abe Lincoln as the sixteenth President of the United States in 1860 led to the eventual breakup of eleven slave-holding states and the formation of the Confederacy. Convinced that the federal government would initiate judicial and legal action against slavery, South Carolina became the first state to quit.
Printed in Charleston, South Carolina, on December 20th, 1860, this broadside announces South Carolinas repeal of the Constitution of the United States and the state's breakup with the Union.
"The Union Is Dissolved!", it asserts.
The Constitution of the new Confederacy would initiate the unrestricted right to hold slaves.