What is the role of a thesis supervisor at the MSc level
A huge role.  Masters is a time to contribute to an established research project while putting in your own experiences to a certain degree.  Usually it is not the Master's students job to come up with a project, understand its research utility/significance etc.  Rather, that project usually exists already and the supervisor asks the student to move it forward, perhaps even support a PhD student in their work.

In this sense, your supervisor should be working closely with you, at least at the beginning, to make sure you understand what needs to be done and how to contribute.  This isn't to say that you're doing exactly what the supervisor says.  Rather, it is understood that a masters student lacks the depth and breadth of understanding of the field required to make a truly impactful, original contribution (usually!  Some are exceptional.  Point is that this isn't the expectation).

By the end of the project you will be the expert in your own contribution.  Your supervisor will guide your thesis and make sure that things like the claims and background section are adequate and truthful.  

You should use your masters studies to explore the field and gain some of the breadth/depth required to understand if it's really the field you'd want to pursue if you were to do PhD.  If that is your goal, you should also use it as an experience to learn the basics of what research is and is not, what a contribution is and is not, and how the life cycle of a research project evolves.  If that is not your goal, then you should also focus on gaining skills and knowledge that you feel will help you in your future career.  Your supervisor's job is to ensure that you are given the freedom to explore these things within the context of the overall project.  

If, for example, you want to learn about linguistics but you are working on a project in international development, that is probably not a good use of you or your supervisor's time unless you can link it somehow.  On the other hand, if you're working on such a project and you decide to study some of the history of the country your project is based in, then that is probably a good use of your time, provided you don't lose track of your main project.  Your supervisor, with regular check-ins, can help you to better understand if you are on the right path with side interests and will also help to steer you back toward your project if you are going too far off track.