Consciousness (mind and mentation) is “the stuff” what we mistake to be subject and object, me and mine, us and things. There is no substantial me, mine, us, things, but only internal representations in our experience.
When you see a movie, the actors and the scenery are not there in the theater. They are images on film, projected on a screen. These images look like various persons and things, but they are all “made” of the same stuff: light and shadow.
In the same way, our every experience is only in and of consciousness, taking the form of (transforming into) subject and object, self and beings, phenomena and their characteristics. Our experience is never extra-mental. It is always internal.