The big issue was the legal rights of slavery and of states in regard to slavery.
In the blanks shown in the comment box by someone, I'd put in the word: S L A V E R Y
Details:
Dred Scott was a slave who had been owned by a Missouri man named John Emerson. Emerson's service in the US military caused him to live for a time in Illinois (which was a free state) and Wisconsin (which was a free territory. Later, back in Missouri, after John Emerson died, Scott tried to purchase his freedom from Emerson's widow, who refused. With lawyers' help, he then filed suit to gain his freedom on the grounds that he had lived in a free state and free territory.
The Supreme Court, by a 7 to 2 margin, issued its ruling that Scott was not entitled to his freedom just because he had lived at some times in free territory. The Court also ruled that African Americans were not and could never be citizens of the United States, and therefore did not have the legal standing to file such a lawsuit. The Court further stated that the Missouri Compromise of 1820 had been an overreach of federal government power -- that any states or territories were free to make their own decisions regarding slavery.
The Dred Scott decision was a controversial event which played into the tensions that by 1861 erupted into the Civil War in the United States.