A dissertation prospectus or proposal describes what or who you plan to research for your dissertation. It delves into why, when, where, and how you will do your research, as well as helps you choose a type of research to pursue. You should also determine whether you plan to pursue qualitative or quantitative methods and what your research design will look like.
It should outline all of the decisions you have taken about your project, from your dissertation topic to your hypotheses and research objectives, ready to be approved by your supervisor or committee.
Note that some departments require a defense component, where you present your prospectus to your committee orally.