A Prospectus is a statement of your plans for a research project.
During the early stages of your research, you should start to make
plans for the finished project.
This writing is the chance to show me that you have
something that is "worth" studying.
The prospectus is used in many others fields, too.
In business, for example, a prospectus is used to show what
the results of a project would be to persuade a possible
investor to give money to "buy into" the business.
So you are trying to get me to "buy into" your idea for the paper.
Below is a link to a page with
more specific information
about how to write and what to say in a Prospectus.