Research prospectus
A prospectus is a researched proposal for a research project. It explains the proposed focus of the paper (i.e. the works or topics that will be covered), the bibliographical context (i.e. important research works that touch on this topic and will useful in writing the paper), the broad outline of the argument that is going to be made and the evidence that is going to be used.
Think of it as a somewhat detailed explanation as to what you are going to write about and why you find it interesting.
A prospectus does not need to be long. 2-4 pages is usually appropriate, though there is no set length.
In grading a prospectus, I will be looking for evidence that you have done some thinking and research on the topic your are proposing, that you are aware of the core primary and some of the main secondary sources that will be useful in your topic, and that you are aware of the broad challenges (if any) involved in the topic you want to write on.
The very best prospectus show evidence of serious thought and reading about the topic in question. Weaker ones show less careful thought, reading, or preparations.