Verses 31-38 are very important. I would summarize the application of them this way:
You can be free and still be a slave.
You can be a slave and still be free.
We tend to define "freedom" as having the ability to do whatever we want. In the US, we have a pretty large amount of freedom according to that definition. Others do not. What Jesus wants us to understand is that how we usually define freedom is not true freedom. True freedom is the ability to be who we were created to be and to bring glory to God through knowing Him and worshiping Him. Jesus offers that. Even though you might be in slavery physically, you can know what true freedom is. The opposite is true: there are millions of "free" people in this world that are trapped by sin and death. Will we offer them freedom?