What style of work is published in your sub-discipline? To that end, a proposal needs to show how your work fits into what is already known about the topic and what new contribution your work will make.
Specify the question that your research will answer, establish why it is a significant question, show how you are going to answer the question, and indicate what you expect we will learn.
(Of course you will have to write the thesis in acceptable form, and you probably will discover things in the course of your research that were not anticipated but which should be addressed in your thesis, but the minimum core intellectual contribution of your thesis will be set by the proposal.) Both parties benefit from an agreed upon plan.
The objective in writing a proposal is to describe what you will do, why it should be done, how you will do it and what you expect will result.
Because they address well-bounded topics, they can be very tight, but they do require more planning on the front end.
Theses which are largely based on synthesis of observations, rumination, speculation, and opinion formation are harder to write, and usually not as convincing, often because they address questions which are not well-bounded and essentially unanswerable.
Different advisors, committees and agencies have different expectations and you should find out what these are as early as possible; ask your advisor for advice on this. (Different advisors will have different preferences about the rules, the meta-discourse, in which we all work.) In the abstract all proposals are very similar.
Further, different types of thesis require slightly different proposals. They need to show a reasonably informed reader why a particular topic is important to address and how you will do it.
In approving the proposal, your committee gives their best judgment that the approach to the research is reasonable and likely to yield the anticipated results.
They are implicitly agreeing that they will accept the result as adequate for the purpose of granting a degree.