In the first stanza, the slave is introduced to us lying next to some un-gathered rice with a sickle in his hand. Among other things, this tells us that he hasn't completed his work for the day. It's not hard to see why. We soon discover that his shirt is open and his hair is matted, indicating that the slave has passed out due to heat exhaustion.
These are the ideal conditions in which he can dream of happier times in Africa, before he was kidnapped and transported thousands of miles across the Atlantic to become a slave in America.
After he stops dreaming, the slave finally gives up the ghost and passes away. Like so many others in his predicament, he has been worked too long and too hard in appalling conditions, and his worn-out, beaten body just couldn't take any more.