This is a great question. A little context is important here.
In this chapter, the Joads are in California and they have a job picking peaches. At first they are happy about the job, since they are in a desparate predicament. They get five cents per box. The family works all day long and they are able to make one dollar. With this money, Ma goes to buy dinner at the local store.
When Ma arrives at the store, she realizes that the store has raised prices. So, not only do the Joads barely make any money for their labor, but also the food costs have risen. They are being hurt on two fronts. In the end, Ma does not have enough to buy food for one meal. And when she wants to buy sugar, the clerk would not give it to her on credit. In this dialogue the clerk who is also poor decides to put up the money himself. When this happens, Ma speaks these unforgettable lines:
"I’m learning one thing good," she said. "Learnin’ it all the time, ever’ day. If you’re in trouble or hurt or need-go to poor people. They’re the only ones that’ll help-the only ones."