Huck fakes his death so that he can escape the clutches of his alcoholic father. Having smashed the door of his cabin and spread pig's blood and some of his hair around, he paddles in his canoe a few miles upstream, sleeps, and ultimately ends up at Jackson Island.
The next morning, having had a nap following his sleep, Huck hears a boom. He realizes a search party is looking for his body. He is hungry, and it occurs to him that
they always put quicksilver in loaves of bread and float them off, because they always go right to the drownded carcass and stop there.
This refers to the superstitious custom of placing quicksilver, another word for mercury, into a loaf of bread in the belief it would float over a corpse submerged in water and, as Huck states, "stop there." This would mark where the body was, and searchers could then easily find it in the water. Unfortunately, there was no scientific basis to this theory.
The practice helps Huck out, however. He is able to retrieve one of the high-quality loaves of floating bread, remove the mercury, and eat it for breakfast. Presumably part of the floating loaf was dry or Huck was hungry enough not to be bothered by wet bread. In any case, Huck enjoys eating this breakfast, smoking his pipe in freedom, and watching the search from behind the island's foliage.