Having an eidetic memory of a dream means that you can see it in your mind so vividly that it's as though you were watching a movie. Beyond memory, the word eidetic is used to describe the essence, or perfect vision, of an object or idea.
The Greek root of eidetic is eidetikos, which means "pertaining to images." While you'll often find the adjective eidetic describing a precise recollection, like a photographic memory, it's used in philosophy and art to describe the essential vision or version of something — e.g., the "essence" of a triangle stripped of color or size. In literature, it might describe writing that evokes images in the mind's eye that are vivid and almost palpable.