A more thorough explanation:

Dred Scott v. Sandford was a U.S. Supreme Court case that ruled that African Americans, whether enslaved or free, were not citizens of the United States and therefore did not have the right to sue in federal court. The Court also ruled that the federal government could not prohibit slavery in the territories. This decision was a major factor leading to the Civil War.

Dred Scott was an African American man who was born a slave in the late 1700s. In 1832, Scott’s owner took him into the Wisconsin territory, which outlawed slavery, to do various tasks. While there, Scott got married, and his owner left him and his wife in Wisconsin when he traveled to Louisiana. Scott attempted to purchase his freedom from his owner's widow, but she refused. Scott then sued in federal court against Sandford, the executor of his owner's estate, for his freedom.

The Supreme Court, in a controversial opinion written by Chief Justice Taney, held that persons of African descent were not citizens of the United States. The Court reasoned that, at the time of the ratification of the U.S. Constitution, persons of African descent were brought to the U.S. as property and could not become U.S. citizens. The Court also found that the federal government could not prohibit slavery in the territories. This decision inflamed tensions between abolitionists and southerners and hastened the coming of the Civil War.

After the Civil War, the Fourteenth Amendment rendered Chief Justice Taney’s entire opinion obsolete by declaring that “all persons born or naturalized in the United States . . . are citizens of the United States.”