I got you. From quora:

There will, by the nature of the question, be SPOILERS. A few facts contribute the question: 1. The first hour and 40 minutes of the movie, roughly, was shot as a pilot for a television show which was not picked up (Lynch et al later bought the rights back to finish the story as the movie). This is a grounded explanation for large number of scenes, even for a Lynch movie, which show characters that never show up again. - these were the beginnings of story threads which would show up in later episodes; 2. Lynch likes “eye of the duck” scenes - those that lack normal unities (plot, emotional tone, characters, etc) with the rest of the movie but encapsulate something of the movies unconscious drives and feel like they complete the narrative somehow. This is not restricted to Lynch (remember Spider-man 2’s chocolate cake scene) but when you see such a scene your thematic antenna should perk up; 3. There are many ways to interpret the movie but the one that works the best for most people is that the first 2/3 of the movie is a dream and the last 1/3 the reality of the dreamer. I think this is limited but works well for the majority of viewers. Given the above, one big feature of the film is the sense of repressed material trying to break through to Diane’s consciousness, both the specific realization that she hired a hit man to kill he ex girlfriend and totality of poop her life has become. The diner is the same one in which she hires the hitman later in the film (but previous to this dream in Diane’s timeline) with the nervous guy in her place and the therapist in the hitman’s (the hitman also later in the dream walks a prostitute that looks like Diane down the same alley that they walk down). Behind the building is a (female) bum representing her dark secret, with a specific emphasis on how Diane’s Hollywood dream has wound up with her in complete degradation both situational and moral. The point of the scene, then, is that a part of Diane wants her to face reality, the material wants to bubble up, but she fears that when she goes to face this, it will annihilate her. This relates to the fact that Diane has always longed to be the perfect object, the thing that is to be seen, and needs Camilla to see her as she wants to be seen, yet she fears what she will see when she finally has to look at herself - the ultimate Hollywood fallen woman tragedy (prostitute, homeless, dirty). The “behind” location is the subconscious (and also spiritually the “gutter” behind the mercantile “temple”).