Dred Scott was a slave who sued for his freedom, based on the principle of location-- he had traveled with his former owner into Illinois and Wisconsin, free states. After a series of appeals, the case ended up going to the Supreme Court.
The Supreme Court ruled that any person of African descent, whether slave or free, was not a citizen of the United States. Furthermore, because Dred Scott was not considered a citizen, he could not bring a a suit to federal court. The Court decision basically said that slaves were property, and the government could not take away a slave without due process or based on where the owner lived ; this decision effectively cancelled out the effectiveness of the Missouri Compromise.
This court decision was most significant because of the 'slaves are property, not citizens' ruling which greatly angered supporters of the Abolition Movement.