Dred Scott was the slave of an army doctor who was posted to a unit in the North. At that point, Scott could have applied for his freedom and it would have been granted automatically, because he was living on free soil. He did not do this, presumably because he was content to work for his master.
Later, they returned to slave country, and when the doctor died, Scott was left to the doctor's family in his will. Scott seems to have resented this, and tried to apply for his freedom retrospectively.
The local judges had never dealt with this kind of application before, and it ended up in the Supreme Court, which rejected it. The Chief Justice surprised and shocked the Abolitionists by ruling that slavery was legal in every state of the Union, because when the Founding Fathers declared that a man's property was sacred, they would have included slaves in their definition of property.
This raised the temperature of the debate, and deepened the divide between the two sections. It is regarded as one of the chief causes of the Civil War.