Let's begin by reviewing the story in question, which is found in Genesis 37. Joseph has majorly annoyed his brothers by telling them all about his dreams, especially since those dreams show that one day all of those brothers will bow down to Joseph. They are also upset that Joseph is their father's favorite, and they are jealous of the gifts their father has given to the young man. One day, their father sends Joseph to go check on his brothers, who are tending the family's flocks. The brothers are so angry by this time that some of them decide to kill Joseph and throw his body into a pit. They plan to tell their father that some wild beast ate him.
This plan does not suit Reuben, however. He doesn't want to actually kill Joseph. He suggests they merely throw him in a pit and secretly thinks that he will rescue Joseph later. The brothers agree to the plan, and into the pit goes Joseph. Pretty soon some traders come along, and Judah suggests that the brothers sell Joseph to them as a slave. Since they mean to get rid of their brother anyway, he says, they might as well make a profit. Reuben is apparently absent during this transaction, and when he goes to look for Joseph in the pit, his brother is gone. He takes the robe Joseph left behind, smears it in goat's blood, and takes it to their father with the story that Joseph was killed by a wild animal. Judah has won that day, and Reuben gets stuck as the messenger of bad news.
This story can, at least in part, serve as symbol of the conflict between the tribes of Israel although its original meaning and historical context should not be discounted. Indeed, the Israelites did live on a fairly small tract of land, and that fact would have led to competition for scarce resources and conflicts between the tribes, just as conflict arose between the original Reuben and Judah. Judah's triumph in the story reflects the triumph of the area settled by his descendants, an area that contains Israel's capital, Jerusalem, which became the center of religion, politics, and commerce over the centuries.