must see

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.