Usually, ‘must’ is a modal verb.
You must see this movie.
You must see the difference. (see can mean understand/notice)
However, ‘must’ can be a noun.
It’s a must. = This is a thing that you must do.
There are new combinations:
As an adjective:
It’s a must-see movie. = It’s a movie that you must see.
As a noun:
That movie is a must-see.