That is a big section of the novel you are talking about, so I have also included some links below to the enotes study guide section of this novel so you can gain more information about what happens.
Having gone up and down the coast of Africa to pick up slaves, the ship readies itself for its voyage across to the Caribbean to sell its cargo. One boy of Jessie's age has particularly struck Jessie, and the two boys form an unspoken bond.
Jessie's job begins and he has to play his fife very day so that the slaves can dance. He is disgusted by this, and one day runs away, refusing to play, and is flogged for disobedience. As the journey continues, the atmosphere becomes more and more claustraphobic and tension mounts. When a slave attacks Nicholas Sparks, and Sparks kills him, Sparks is himself bound and thrown overboard. He after all has lost the captain some profit.
Stout tries to make friends with Jessie, but when Jessie rebuffs all atempts, he puts his fife in the hold with the slaves and makes Jessie get it as a punishment. When he goes down there, the boy has found it for him and gives it to him. The captain starts negotiating with a Spanish ship off the coast of Cuba and will sell the slaves.
As the sailors celebrate by sinking in to drunken revelry, an American ship comes alongside. Slaves are thrown overboard and the two ships fight. Jessie and the boy stay together locked in the hold for several days. They are finally able to escape and swim to shore. The two boys are taken in by an old man who looks after them and arranges for the boy to be taken to freedom. He tells Jessie how to return to New Orleans after making him promise to not talk about his friend.
Jessie makes it home and describes his reunion with his family and gives a brief summary of the rest of his life. When the Civil War breaks out he fights on the Northern side. Even as he gets older, he is still never able to forget the sight of the slaves dancing to his music.