Jonas is Teta Elzbieta's brother. He is also the one who first brings the whole idea of "America" into the family's discussions back in Lithuania. He has an old buddy, Jokubas Szedvilas, who (Jonas thinks) has struck it big in Chicago's meatpacking district. On the strength of this news, Jurgis and the rest of the family decide to head to the United States. Jonas wants to get rich, and he soon comes to realize that he can't do that working his dead-end job pushing carts around the packing plants and using all of his wages to pay for the mortgage on a house no one likes. So, one day, he picks up his weekly salary and takes off. No one in the family sees or hears from him again. They wonder briefly if he might be dead, but everyone agrees that it's much more likely that he is fed up.
Jonas's disappearance is the first true financial disaster the family has, because now they don't have his contribution to the mortgage. Even though it might be understandable that Jonas has left – we can see why he is frustrated with his awful family life – his selfishness really leaves the Jurgis family in a pickle. Once Jonas takes off, Stanislovas has to leave school and start working at a factory, which is truly tragic. So again, we see the selfishness that Sinclair suggests the profit motive breeds in people. Jonas's decision to abandon the family only foreshadows Jurgis's much more dire departures later on in the novel.