Review of The Mirror (1975)
IMDb
Andrei Tarkovsky (director).
Your guess is as good as mine. The general consensus seems to be that a dying soldier recalls his life.
Art with heavy focus on the audiovisual, but also incorporating a thin plot co-written by the director, and extradiegetic poetry by the director’s father. The formalists of the 1920s, with their ideas about montage and so forth, could have used this single film to greatly expand upon the mystical property they called “the cinematic”.