Several important things happen in this chapter, and if you get a chance, I recommend reading it; it's not very long, and as all of the book, a very entertaining read.
First, Huck sees a ferry on the river that has "Pap, and Judge Thatcher, and Bessie Thatcher, and Joe Harper, and Tom Sawyer, and his old Aunt Polly, and Sid and and Mary, and plenty more" on it, and they are looking for Huck's body. One way to do that is to shoot a cannon out over the river; the vibrations from the cannon-shot will hopefully "make [Huck's] carcass come to the top" of the water, if it was lodged in something, or sunk. A bit morbid, but it makes sense. Secondly, they sent out loaves of bread with quicksilver in the middle of them, because supposedly "they always go right to the drownded carcass and stop there." Huck doesn't pass up the opportunity for free bread, so he fishes out a loaf and eats it. Then, he explores a bit and as he does, he runs across Jim, Miss Watson's slave. He informs Huck that he has run away because Miss Watson was going to sell him to New Orleans, away from his family. So, he took off. Huck promises not to tell anyone, and to help him.
Those are just a few important things, and I hope that helps! Good luck!