"that he was sick..."
See in text (Chapter II)
Linda expresses her disbelief that her father could have died so suddenly without her knowledge of his sickness. Many people take this to mean that he was killed at the hands of his master, most likely beaten to death for misbehaving. This explains why he wasn’t sick and why Linda was not allowed to see his body after he had died. A possible reason for his death is revealed later on, he “spoiled his children, by teaching them to feel that they were human beings.”
See in text (Chapter VI)
A “bloodhound” is a breed of dog with a keen sense of smell, originally bred for hunting deer, boar, and people. Here, Linda calls northerners “bloodhounds” to insinuate that they hunt runaway slaves like dogs—a reference to the Fugitive Slave Act of 1850, which required all escaped slaves found in the North to be sent back to their owners. Abolitionists even nicknamed this act the “Bloodhound Act” because of the hunting that came from it.
"Mason and Dixon's line..."
See in text (Chapter VIII)
The Mason-Dixon line is the boundary between Pennsylvania and Maryland. Originally created to settle a land dispute in the 1760s, the Mason-Dixon line became important during the Civil War when it became a symbol of the separation between Northern “Free States” and Southern “Slave States”.