Your dissertation proposal might be in the form of a proof paper or proof dissertation. In this case, you are proposing that your dissertation will prove something significant. This must be related to your degree. Perhaps you are going to prove that labor unions facilitate better wages, or that the electoral college is flawed. Whatever you choose to prove in your proposal, you must do the research or experimentation necessary to prove it in your dissertation.
Your dissertation might be an argument, and in that case your proposal will outline the argument and how you plan to discuss it. You might argue for or against banning books in schools, or argue for or against the death penalty. The argument should relate in some way to your field of study, and should reflect on the things you have learned during your program thus far.
Some dissertations are an outlining and proof of a theory, which the proposal would reflect. You might have come up with a new scientific theory, perhaps about space travel or animal husbandry. Your proposal should outline this theory and state your belief, as well as providing the backup information you have to prove the theory is correct.
Some dissertations are research papers. If you are getting a degree in English or History, you may do a research dissertation. Your proposal would reflect the topic, such as Hitler, Shakespeare or the history of America, and should outline how you plan on using the knowledge that you've obtained to complete the dissertation.