Chapters 21-26: What Jail Taught a Preacher

How I interpret what Casy is saying is that even though there are consequences for doing something that is vital for survival and no one may not care to listen to your reasoning, by speaking up enough, eventually someone is going to hear you and come along and make a change. Casy realized during that time that there are a lot of things that people constitute as needs, but when you break it down, there are so few things that we truly need in life, but we all just get so caught up in living a grand life that we tend to miss what is really important. After this conversation, it gives a better understanding why Casy lives such a simple life and does not see the point in having all sorts of fancy things because he does not need them to survive. 
While he is in jail, Casy and the inmates are served rotten beans for their meal. One prisoner is not happy with this and complains. Nothing happens, go figure. But this one prisoner standing up for something gave others courage to stand up and complain and yell. It is an obvious theme throughout this book that there is strength in numbers. With every fiber of his being, Casy believes that if all of the migrant workers stand up to the farming companies about their low wages and minimal hiring, then they can make a change.
These kind of things do not just happen in novels; they happen all over the world. Take a journey to the other side of the globe and look at the revolts in countries such as Egypt and Libya. In Egypt, the citizens (mostly teenagers and young adults) had had enough of the government and decided to do something about it. The government responded by blocking social networking sites in order to prevent people from spreading the news of their rallies. Fortunately, it is impossible to block the internet from the entire world, so news got around and in the end, the people won.