The Supreme Court essentially took up three questions in this case.
First, it considered whether Dred Scott had any standing to sue. The Court ruled that he had no standing and therefore no right to even bring the case. It ruled that African Americans were not citizens under the Constitution because the people who wrote the Constitution had no intention of giving blacks the same rights as whites. Because of that, Scott did not have the right to sue.
Second, the Court addressed the idea that a slave could be set free simply by being brought into a free state or territory. The Court rejected this idea. Scott might have been entitled to be free while he was in a free state, but once he went back to a slave state, he no longer had any right to claim that he was free.
Finally, the Court asked whether Congress could make laws about slavery in the territories. The Court said that Congress could not do that because such laws violated the Fifth Amendment. Such laws took property (slaves) away from owners without due process of law.