A prospectus (in this class, at least) is a preliminary statement for a paper. It has two purposes - first, to get you to think about your paper before the night before it is due, and second, to let me see what you have in mind so I can help you develop the question you want to ask, and direct you to some resources.
The prospectus is brief - a page or so - and contains some common elements:
1. The prospectus needs to contain the topic, the question, and the thesis of the paper. What's the difference?
Topic: The topic is the general area you want to write about. Examples of topics are things like: "AIDS in America", "The causes of the WWI", "Dali". Topics tend to be broad, and are like the "field" in which questions are asked.
Question: Topics are not questions. You ask a question within the area of the topic. One topic could have many questions associated with it. For example, the topic "AIDS in America" could have the following questions: "What is the history of AIDS in America?", "What is the relationship between religion and AIDS in America?", "What are some of the ways that the trauma of AIDS has been communicated through art or music?", and so forth. You can see that there could be hundreds of possible questions.
When you get a question, or a few possible questions, you should analyze them. Define the words, even the easy ones. Think about other ways to ask the same question (that is, reword it). Do whatever it takes to become as clear as possible about the question you are asking.
It is important to learn to recognize the kinds of questions asked in particular disciplines. In the humanities, we tend to move between and across disciplines, but that doesn't mean that questions don't draw on the resources of disciplines. Philosophical questions are usually different from historical or psychological ones
Thesis: The thesis is just the answer to the question. Good theses come from good questions, poor theses come from unclear questions. A great many papers are just a collection of vaguely relevant information to a topic. They do not know what they are asking, and therefore they don't know what a good answer would look like.
2. The prospectus needs to make clear how you are going to answer your question, or how you are going to defend your thesis (that's two ways of saying the same thing). If you have a good question, it should be clear how you need to go about answering it. This just needs to be a short outline of the argument you are going to use.
3. The prospectus needs to have a bibliography. You should give me an idea of the relevant sources you will use to write the paper. These should include scholarly sources.
The final paper may deviate from the prospectus. This often happens (although, if you want to do a completely new topic, you should see me about it). Sometimes, the question you think you can ask turns out to not be a good one, or not be answerable, or answerable only with a doctoral dissertation, or vague, or uninteresting. You wouldn't know any of this if you started the paper the day before, as is usually the case. A prospectus allows some second thought, which is also a good idea in writing a paper.